drm/i915: Keep physical cursors pinned while in use

The optimisation inherent in commit 6a2c4232 ("drm/i915: Make the
physical object coherent with GTT") relies on that once we allocated a
cursor we would have coherent, zero overhead access to the scanout plane
holding the cursor. That is we could then do the very frequent cursor
updates X enjoys with no indirection or kernel involvement. However,
that all hinges on the GGTT mmap of the cursor being pinned and not
require refaulting on each access -- handling such a page fault likely
requires the busy GGTT to be rearranged causing a stall. A very simple
fix is then to handle the physical cursor exactly like other cursors and
keep its vma pinned while active.

References: https://bugs.freedesktop.org/show_bug.cgi?id=107600
References: 6a2c4232

 ("drm/i915: Make the physical object coherent with GTT")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180817082405.755-1-chris@chris-wilson.co.uk