drm/amd/display: Mask DPCD field to avoid reading unsupported field value

[Why]
DSC will be disabled if DPCD 0006F[6:3] is set to a non-zero value
because bits 6:3 are not currently supported. When 6:3 is populated, an
unsupported INCREMENT OF bits_per_pixel value is read (DPCD 0006F[2:0])

[How]
Mask the INCREMENT OF bits_per_pixel field so that values in the
unsupported field are ignored.

Signed-off-by: Jaehyun Chung <jaehyun.chung@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>