drm/i915: Tidy slab cache allocations

drm/i915: Tidy slab cache allocations

We can use the preferred KMEM_CACHE helper for brevity.

Also simplifiy error unwind by only setting the ENOMEM
error code once.

v2: Add forgotten changes. (Joonas Lahtinen)

Signed-off-by: Tvrtko Ursulin <[email protected]>
Reviewed-by: Joonas Lahtinen <[email protected]> (v1)
Cc: Joonas Lahtinen <[email protected]>
Reviewed-by: Joonas Lahtinen <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]