drm/amd/display: Fix eDP not light up on resume

[why]
Only on VG, if external display is disconnected during S3 suspend, the
internal panel doesn't light up on resume because we set the power state
using an unsupported DPCD register SET_POWER.  To check the register is
supported, we need to check SET_POWER_CAPABLE first which is
eDP-specific DPCD register field.

[how]
Check the SET_POWER_CAPABLE register field and decide the control of the
eDP power state based on the read register value.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Agustin Gutierrez <Agustin.Gutierrez@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Sung Joon Kim <sungkim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>