drm/amd/display: refactor dmub commands into single function

[Why & How]
Consolidate dmub access to a single interface. This makes it easier to
add code in the future that needs to run every time a dmub command is
requested (e.g. instrumentation, locking etc).

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Josip Pavic <Josip.Pavic@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>