drm/amd/display: Use pitch when calculating size to cache in MALL

[Description]
Use pitch when calculating size to cache in MALL

Reviewed-by: Samson Tam <Samson.Tam@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>