drm/atomic: Handle 64-bit return from drm_crtc_vblank_count()

570e8696

 ("drm: Widen vblank count to 64-bits [v3]") changed the
return type for drm_crtc_vblank_count() to u64.

The flip ioctl receives a 32-bit target sequence from user space and is
compared against the current sequence from drm_crtc_vblank_count(). So,
typecast return from drm_crtc_vblank_count() explicitly to add clarity.

__drm_crtcs_state.last_vblank_count however only ever stores the value from
drm_crtc_vblank_count() and can be upgraded to u64.

Cc: Keith Packard <keithp@keithp.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180203051302.9974-7-dhinakaran.pandiyan@intel.com