drm/amd/display: Fix ABM pipe/backlight issues when change backlight

[Why]
set ABM pipe/backlight gets some issues when abm callback func pointers
are NULL. For some usecase, driver would like to control PWM level before
ABM resource is ready. However, recent flow refactor of ABM didn't
consider that use case.

[How]
Rollback flow that sending inbox command to dmub directly when ABM
function pointers aren't ready.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Leon Huang <Leon.Huang1@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>