drm/i915: Update missing parts after the rename to i915_request

Mostly doc/print messages that were not updated after commit e61e0f51
("drm/i915: Rename drm_i915_gem_request to i915_request").

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180222172405.11386-1-michel.thierry@intel.com