transcode: fix VAAPI deinterlace mode handling for software decode/encode profiles

transcode: fix VAAPI deinterlace mode handling for software decode/encode profiles

Move 'deinterlace_vaapi_mode' from the VAAPI codec profile to the generic Main
Video Codec Profile, allowing this setting to be applied when using VAAPI
hardware deinterlacing with software-based transcode profiles such as libx264.

This fixes a bug where 'deinterlace_vaapi_mode' was left uninitialized for
non-VAAPI codec profiles, resulting in invalid filter strings like
'deinterlace_vaapi=mode=21867:rate=2:auto=0' and filter graph setup failures.

The patch also:
- Adds validation for the mode value (range 0–4)
- Dynamically enables/disables the VAAPI mode field in the WebUI based on encoder
  and decoder settings
- Consolidates enum and mode list generation logic under 'profile_video_class.c'

This ensures that VAAPI deinterlacing can be correctly configured and used across
hybrid transcode profiles, improving compatibility and user control.

Fixes: #1878