drm/amdgpu: Set vblank_disable_allowed = true

Without this, since the conversion from drm_vblank_pre/post_modeset to
drm_vblank_on/off, the vblank interrupt could never be disabled after
userspace triggered enabling it.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>