drm/vblank: Switch to bool in_vblank_irq in get_vblank_timestamp

It's overkill to have a flag parameter which is essentially used just
as a boolean. This takes care of core + adjusting drivers.

Adjusting the scanout position callback is a bit harder, since radeon
also supplies it's own driver-private flags in there.

v2: Fixup misplaced hunks (Neil).

v3: kbuild says v1 was better ...

Cc: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Cc: Eric Anholt <eric@anholt.net>
Cc: Rob Clark <robdclark@gmail.com>
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170509140329.24114-2-daniel.vetter@ffwll.ch