drm/amdgpu: Define one variable for virtual display.

For virtual display feature, define on variable in amdgpu.ko. When want to
enable virtual display feature, need set the option "amdgpu.virtual_display=1".
And then disable vga render and crtc if have DCE engine.

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>