Move equal_dimensions_and_border to yv12config.h

[Arjit201]

Summary

Move equal_dimensions_and_border() to avm_scale/yv12config.h and remove the duplicate implementations from:

• av2/decoder/decoder.c
• av2/encoder/encoder_utils.h

Closes #5030

This consolidates the helper into a single shared definition associated with YV12_BUFFER_CONFIG and eliminates duplicate code.

Testing

• Verified only a single definition remains via git grep.
• Existing call sites continue to use the shared helper.

[urvangjoshi]

@Arjit201 : Thanks for your first contribution here! Pls sign Contributor licence agreement (CLA) here: https://aomedia.org/about/legal/
Let me know when you have done so.

[urvangjoshi]

Pls format your code following https://github.com/AOMediaCodec/avm/wiki/Reproducing-CI-Test-Failures-Locally#style-check