[gemma3] Fix do_convert_rgb in image processors.
Author: MohitIntel
Pull request: yes

* Adds the do_convert_rgb=True statement in image_processing_gemma3_fast.py as suggested in code review for https://github.com/huggingface/transformers/pull/39375
This PR in conjunction with https://github.com/huggingface/transformers/pull/39376 and https://github.com/huggingface/transformers/pull/39375 is needed to make lm-eval run on gemma3.  
See https://github.com/huggingface/transformers/pull/39376 for more details on the command used.

* Fix typo in comments of do_pan_and_scan input variable.

Fixes # https://github.com/huggingface/transformers/issues/23447

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Comments:
cc @zucchini-nlp 

Ensures correct default value for do_convert_rgb parameter is set in processing_gemma3.py otherwise lm-eval on the MMMU task with this model results in "ValueError: Unable to infer channel dimension format".

```
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
```


CI should be green now!

**[For maintainers]** Suggested jobs to run (before merge)

run-slow: gemma3

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