drm/amdgpu: Fix dead lock issue for vblank

Always start vblank timer, but only calls vblank function
when vblank is enabled.

This is used to fix the dead lock issue.
When drm_crtc_vblank_off want to disable vblank,
it first get event_lock, and then call hrtimer_cancel,
but hrtimer_cancel want to wait timer handler function finished.
Timer handler also want to aquire event_lock in drm_handle_vblank.

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