drm: fix typo in drm_gem_get_pages() comment

I spent an embarrassingly long time looking for drm_gem_init_object()
before I realized I was actually looking for drm_gem_object_init().
Fix the typo to keep other poor developers from suffering the same
fate.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1507045091-6550-1-git-send-email-jcrouse@codeaurora.org