drm/i915: Use the right mapping_gfp_mask for final shmem allocation

Many sightings report the greater prevalence of allocation failures.
This is all due to the incorrect use of mapping_gfp_constraint(), so
remove it in favour of just querying the mapping_gfp_mask() which are
the exact gfp_t we wanted in the first place.

We still do expect a higher chance of reporting ENOMEM, as that is the
intention of using __GFP_NORETRY -- to fail rather than oom after having
reclaimed from our bo caches, and having done a direct|kswapd reclaim
pass.

Reported-by: Jason Ekstrand <jason.ekstrand@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100594


Fixes: 24f8e00a ("drm/i915: Prefer to report ENOMEM rather than incur the oom for gfx allocations")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170405221514.23251-1-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>