drm/amdgpu: Add amdgpu "max bpc" connector property (v2)

[Why]
Many panels support more than 8bpc but some modes are unavailable while
running at greater than 8bpc due to DP/HDMI bandwidth constraints.

Support for more than 8bpc was added recently in the driver but it
defaults to the maximum supported bpc - locking out these modes.

This should be a user configurable option such that the user can select
what bpc configuration they would like.

[How]
This patch introduces the "max bpc" amdgpu driver specific connector
property so the user can limit the maximum bpc. It ranges from 8 to 16.

This doesn't directly set the preferred bpc for the panel since it
follows Intel's existing driver conventions.

This proprety should be removed once common drm support for max bpc
lands.

v2: rebase on upstream (Alex)

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>