reservation: fix small comment typo

Signed-off-by: Rob Clark <robdclark@gmail.com>
[danvet: Mark up as function for proper cross-linking.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1471640134-30888-1-git-send-email-robdclark@gmail.com