drm: Trivial comment grammar cleanups

Most of these are just cases where code comments used contractions
(it's, who's) where they actually mean to use a possessive pronoun (its,
whose) or vice-versa.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190202012326.20096-1-matthew.d.roper@intel.com