drm/amdgpu: fix set scaling mode Full/Full aspect/Center not works on vga and dvi connectors

amdgpu_connector_vga_get_modes missed function amdgpu_get_native_mode
which assign amdgpu_encoder->native_mode with *preferred_mode result in
amdgpu_encoder->native_mode.clock always be 0. That will cause
amdgpu_connector_set_property returned early on:
if ((rmx_type != DRM_MODE_SCALE_NONE) &&
	(amdgpu_encoder->native_mode.clock == 0))
when we try to set scaling mode Full/Full aspect/Center.
Add the missing function to amdgpu_connector_vga_get_mode can fix this.
It also works on dvi connectors because
amdgpu_connector_dvi_helper_funcs.get_mode use the same method.

Signed-off-by: hongao <hongao@uniontech.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org