drm/i915: i915_gem_alloc_context_obj can be static

It has only one call site from the same file.

v2: Also rename it to alloc_context_obj. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1479898155-21014-1-git-send-email-tvrtko.ursulin@linux.intel.com