drm/amdgpu: Add amdisp pinctrl MFD resource

AMDISP GPIO control uses a dedicated pinctrl driver,
and requires MFD hotadd GPIO resources.

Co-developed-by: Pratap Nirujogi <pratap.nirujogi@amd.com>
Signed-off-by: Benjamin Chan <benjamin.chan@amd.com>
Signed-off-by: Pratap Nirujogi <pratap.nirujogi@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>