drm/amd/display: remove outdated 8bpc comments

[Why]
The commit c76e483c ("drm/amd/display: Don't restrict bpc to 8 bpc")
removes the historical 8bpc dependency and sets max_bpc to 16.

[How]
The comment that states "8bpc for non-edp" needs to be removed as well.

Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>