drm/i915: Allocate the proper size for contexts.

Whoops. This was fixed previously, but not sure how it got lost. It's
not needed for -fixes or stable because at the moment
drm_i915_file_private is way bigger than i915_hw_context (by 120 bytes
on my 64b build).

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>