drm/i915: Only save/restore cursor regs if !KMS

Under KMS, restoring the cursor is handled upon modeswitch in order to
avoid enabling an undefined set of registers. At the moment, the cursor
is restored before the aperture and modes are fully setup causing some
invalid access during resume, such as:

  PGTBL_ER: 0x00040000
    Invalid GTT entry during Cursor Fetch

Fix this by only performing cursor register save/restore under UMS where
it is done in the correct sequence.

Reported-by: Arkadiusz Miskiewicz <arekm@maven.pl>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>