drm/amd/display: change from panel to panel cntl

[Why]
it doesn't represent panel specifically, it's more like the control
logic for the panel

[How]
change from panel to panel cntl to make it a bit more clear

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