drm/amd/display: Add function pointers for panel related hw functions

[Why]
Make panel backlight and power on/off functions into
hardware specific function pointers

[How]
Add function pointers for panel related hw functions
 - is_panel_powered_on
 - is_panel_backlight_on

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>