drm/amdgpu/display: remove VEGAM config option

Leftover from bringup.  No need to keep it around for
upstream.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>