drm/amd/display: move eDP panel control logic to link_edp_panel_control

Create new file link_edp_panel_control.c and link_edp_panel_control.h,
and move eDP panel control logic into them.

Reviewed-by: George Shen <George.Shen@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>