drm/qxl: Fix qxl_noop_get_vblank_counter()

This breaks under the vblank timestamp cleanup patch
by Daniel Vetter. Also it is pointless to return anything
but zero (or any other constant) if the function doesn't
actually query a hw vblank counter. The bogus return of
the current drm vblank counter via direct readout or via
drm_vblank_count() is found in many of the new kms drivers,
but it does exactly nothing different from returning any
arbitrary constant - it's a no operation.

Let's simply return 0 - Easy and fast.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>