drm/i915: fix spelling mistake "reserverd" -> "reserved"

Fix a spelling mistake in a comment.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181120151415.32419-1-alexandre.belloni@bootlin.com