drm/i915: Remove two sloppy inline functions from .h

Get rid of sloppy inline functions now that we don't have more users:

i915_gem_request_get_seqno
i915_gem_request_get_engine

v2:
- request->engine is always non-NULL (Chris)

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1478589108-3702-1-git-send-email-joonas.lahtinen@linux.intel.com