drm/i915: Prefer to report ENOMEM rather than incur the oom for gfx allocations

Since gfx allocations tend to be large, unmovable and disposable, report
the allocation failure back to userspace as an ENOMEM rather than incur
the oomkiller. We have already tried to make room by purging our own
cached gfx objects, and the oomkiller doesn't attribute ownership of gfx
objects so will likely pick the wrong candidate. Instead, let userspace
see the ENOMEM.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170322110521.29930-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>