drm/vblank: Do not update vblank count if interrupts are already disabled.

Updating vblank counts requires register reads and these reads may not
return meaningful values if the device was in a low power state after
vblank interrupts were last disabled. So, update the count only if vblank
interrupts are enabled. Secondly, this means the registers should be read
before disabling vblank interrupts.

v2: Don't check vblank->enabled outside it's lock (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180203051302.9974-8-dhinakaran.pandiyan@intel.com