drm/amd/display: Exit from idle state before accessing HW data

[why & how]
User interface cannot guarantee system is in
idle state, so need to ensure we exit idle state
before accessing any HW data.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Sung Joon Kim <sungkim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>