Set do_convert_rgb default in processing_gemma3.py
Author: MohitIntel
Pull request: yes

# What does this PR do?
Ensures correct default value for `do_convert_rgb` parameter is set otherwise lm-eval on this model results in "ValueError: Unable to infer channel dimension format".
This patch is needed in conjunction with  https://github.com/huggingface/transformers/pull/39375 for lm-eval to work on this model with the following command:
```
lm_eval --model vllm-vlm --model_args '{"pretrained":"google/gemma-3-27b-pt","tensor_parallel_size":1,"distributed_executor_backend":"mp","trust_remote_code":true,"max_model_len":4096,"use_v2_block_manager":true,"dtype":"bfloat16","max_num_seqs":128,"limit_mm_per_prompt":{"image":10}}' --tasks mmmu_val_economics --batch_size 128 --num_fewshot 8
```
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Fixes # (issue)


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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: gemma3

Closing this PR as all related changes are now in newer PR ready for review : https://github.com/huggingface/transformers/pull/39438