drm: Update obsolete information from {enable/disable}_vblank hooks.

Since commit 4dfd64862ff8 ("drm: Use vblank timestamps to guesstimate
how many vblanks were missed"), the DRM framework can cope with devices
that don't have a hardware counter for vsync events without having
to keep the vsync interrupts enabled all the time. Drivers handling
such hardware should use drm_vblank_no_hw_counter() function for
their ->get_vblank_counter hook.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464795342-32297-1-git-send-email-Liviu.Dudau@arm.com