mtmd: refactor llava-uhd overview image handling (always use ov_img_first)

[ngxson]

Overview

Important

if you get the error "no image tokens produced by preprocessor", comment below the model you are using (link to GGUF file) as well as the input image size that you are using

llava-uhd overview image logic is a bit messy, because some models hard-coding the handling inside preprocessor, while the other rely on ov_img_first

With this PR, ov_img_first is now the single source of truth.

Test results:

[vision] OK:   ggml-org/SmolVLM-500M-Instruct-GGUF:Q8_0
[vision] OK:   ggml-org/SmolVLM2-2.2B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/SmolVLM2-500M-Video-Instruct-GGUF:Q8_0
[vision] OK:   ggml-org/gemma-3-4b-it-GGUF:Q4_K_M
[vision] OK:   THUDM/glm-edge-v-5b-gguf:Q4_K_M
[vision] OK:   second-state/Llava-v1.5-7B-GGUF:Q2_K
[vision] OK:   cjpais/llava-1.6-mistral-7b-gguf:Q3_K_M
[vision] OK:   ibm-research/granite-vision-3.2-2b-GGUF:Q4_K_M
[vision] OK:   second-state/MiniCPM-Llama3-V-2_5-GGUF:Q2_K
[vision] OK:   openbmb/MiniCPM-V-2_6-gguf:Q2_K
[vision] OK:   openbmb/MiniCPM-o-2_6-gguf:Q4_0
[vision] OK:   bartowski/Qwen2-VL-2B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/Qwen2.5-VL-3B-Instruct-GGUF:Q4_K_M
[vision] OK:   ggml-org/InternVL2_5-1B-GGUF:Q8_0
[vision] OK:   ggml-org/InternVL3-1B-Instruct-GGUF:Q8_0
[vision] OK:   ggml-org/Qwen2.5-Omni-3B-GGUF:Q4_K_M
[vision] OK:   ggml-org/LFM2-VL-450M-GGUF:Q8_0
[vision] OK:   ggml-org/granite-docling-258M-GGUF:Q8_0
[vision] OK:   ggml-org/LightOnOCR-1B-1025-GGUF:Q8_0
[vision] OK:   ggml-org/DeepSeek-OCR-GGUF:Q8_0
[vision] OK:   ggml-org/dots.ocr-GGUF:Q8_0
[vision] OK:   ggml-org/HunyuanOCR-GGUF:Q8_0
[vision] OK:   ggml-org/gemma-4-E2B-it-GGUF:Q8_0
[audio]  OK:   ggml-org/ultravox-v0_5-llama-3_2-1b-GGUF:Q8_0
[audio]  OK:   ggml-org/Qwen2.5-Omni-3B-GGUF:Q4_K_M
[audio]  OK:   ggml-org/Voxtral-Mini-3B-2507-GGUF:Q4_K_M
[audio]  OK:   ggml-org/LFM2-Audio-1.5B-GGUF:Q8_0
[audio]  OK:   ggml-org/gemma-4-E2B-it-GGUF:Q8_0
[audio]  OK:   ggml-org/Qwen3-ASR-0.6B-GGUF:Q8_0

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: minor corrections provided by glm-5.1

[ngxson]

@sfallah this should be merged before #24717 , please take a look

[ngxson]

nvm, this seems good, I'm merging it now to unblock the other PR

=== Summary ===
  DeepSeek-OCR -- single-view scan                 PASS
  DeepSeek-OCR-2 -- single-view scan               PASS
Overall: PASS