drm/i915: Unset cursor if out-of-bounds upon mode change (v4)

The docs warn that to position the cursor such that no part of it is
visible on the pipe is an undefined operation. Avoid such circumstances
upon changing the mode, or at any other time, by unsetting the cursor if
it moves out of bounds.

"For normal high resolution display modes, the cursor must have at least a
single pixel positioned over the active screen.” (p143, p148 of the hardware
registers docs).

Fixes:

  Bug 24748 - [965G] Graphics crashes when resolution is changed with KMS
              enabled
  https://bugs.freedesktop.org/show_bug.cgi?id=24748

v2: Only update the cursor registers if they change.
v3: Fix the unsigned comparision of x,y against width,height.
v4: Always set CUR.BASE or else the cursor may become corrupt.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reported-by: Christian Eggers <ceggers@gmx.de>
Cc: Christopher James Halse Rogers  <chalserogers@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Eric Anholt <eric@anholt.net>