drm/i915: Do a bit of cleanup in the sprite code

Move the variable initialization to where the variables are declared,
and kill a pointless to_intel_crtc() cast when we already have the
casted pointer.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>