drm: Remove internal setup of struct drm_device.vblank_disable_immediate

drm: Remove internal setup of struct drm_device.vblank_disable_immediate

VBLANK interrupts can be disabled immediately or with a delay, where the
latter is the default. The former option can be selected by setting
get_vblank_timestamp and enabling vblank_disable_immediate in struct
drm_device. Simplify the code in preparation of the removal of struct
drm_device.get_vblank_timestamp.

v3:
	* remove internal setup of vblank_disable_immediate

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123135943.24140-2-tzimmermann@suse.de