drm/i915/cmdparser: Remove stray intel_engine_cs *ring

drm/i915/cmdparser: Remove stray intel_engine_cs *ring

When we refer to intel_engine_cs, we want to use engine so as not to
confuse ourselves about ringbuffers.

v2: Rename all the functions as well, as well as a few more stray comments.
v3: Split the really long error message strings

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-6-git-send-email-chris@chris-wilson.co.uk
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469606850-28659-1-git-send-email-chris@chris-wilson.co.uk