drm/amd/display: Fix unintialized max_bpc state values

[Why]
If the "max bpc" isn't explicitly set in the atomic state then it
have a value of 0. This has the correct behavior of limiting a panel
to 8bpc in the case where the panel supports 8bpc. In the case of eDP
panels this isn't a true assumption - there are panels that can only
do 6bpc.

Banding occurs for these displays.

[How]
Initialize the max_bpc when the connector resets to 8bpc. Also carry
over the value when the state is duplicated.

Bugzilla: https://bugs.freedesktop.org/108825
Fixes: 307638884f72 ("drm/amd/display: Support amdgpu "max bpc" connector property")

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