drm/i915: Only update i845/i865 CURBASE when disabled (v2)

The i845 and i865 have a peculiarlity in that CURBASE is not the trigger
for the vsync update of the cursor registers but instead the
modification of that register is prohibited whilst the cursor is
enabled. Reorder the write sequence for CURPOS, CURCNTR and CURBASE on
i845 to i865 to match.

v2: Remove the checks for i845/i865 from within i9xx_cursor_update()

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>