drm/i915: Rename gen8_(un)request_engine_reset to gen8_reset_engine_start/cancel

drm/i915: Rename gen8_(un)request_engine_reset to gen8_reset_engine_start/cancel

As all other functions related to resetting engines are using
reset_engine.

v2: remove _request_ and use start/cancel instead (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170418202335.35232-3-michel.thierry@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>