drm/amd/display: Correctly restore user_level

[Why]
BL1_PWM_USER_LEVEL is meant for the user brightness level setting from
OS. However, we update it along with other ABM levels to the real PWM
value which could be ABMed.

[How]
Driver to cache and restore the user brightness level setting so that
DMUB can retrieve the last user setting in ABM config initialization.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Anthony Koo <anthony.koo@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Camille Cho <camille.cho@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>