drm/i915: Fix cursor updates on some platforms

Turns out that just writing CURPOS isn't sufficient to move the cursor
on some platforms. My 830 works just fine, but eg. 945 and PNV don't.
On those platforms we need to arm even the CURPOS update with a
CURBASE write.

Even worse, a write to any of the cursor register apart from CURBASE
will cancel an already pending cursor update. So if we have armed a
CURCNTR/CURBASE update, a subsequent CURPOS write prior to vblank
would cancel that armed update. Thus we're left with a cursor that
doesn't appear to move, or even change shape.

Fix the problem by always performing the CURBASE write after a
CURPOS write. Bspec is somewhat unclear which platforms actually
require this CURBASE write and which don't. So to keep it simple
and to make sure we really fix the problem across all supported
devices, let's just perform the CURBASE write unconditionally.

Cc: Paul Menzel <pmenzel@molgen.mpg.de>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101790
Fixes: 75343a44

 ("drm/i915: Drop useless posting reads from cursor commit")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Paul Menzel <paulepanter@users.sourceforge.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170714155227.6089-1-ville.syrjala@linux.intel.com