drm/i915: Move the cursor_base setup to i{845, 9xx}_update_cursor()

To make the code a bit more undestandable move the
intel_crtc->cursor_base assignment into the low level update cursor
routines. That's were we compare the current value with the new one
so immediately seeing that it gets assigned only afterwards helps
one to understand that it gets assigned only after the comparison.

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>