drm/amd/display: fix bug in the logic for panel power control

[Why]
there's a bug in the new logic for panel power control.  the check is
wrong, and will skip panel power control under the wrong conditions.

[How]
fix to check for NULL panel_cntl

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Ashley Thomas <Ashley.Thomas2@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>