drm/amd/display: Add dc cap to restrict VSR downscaling src size

- Adds int max_downscale_src_width in dc struct
- Checks and does not support if downscale size is more than 4k (width > 3840)

Signed-off-by: Xingyue Tao <xingyue.tao@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>