drm/amd/display: Make FBC work without fbdev emulation

Previously, FBC dynamic allocation relied on connector modes
populated during dm init. This is only the case if
DRM_FBDEV_EMULATION config flag is enabled.
Moving fbc allocation from dm_late_init() to
amdgpu_dm_connector_get_modes() where actual modes init happens.

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Shirish Shankarappa <Shirish.S@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>