drm: Fix a typo in drm_ioctl.c

This patch fix a spelling typo found in
Documentation/DocBook/gpu/API-drm-ioctl-flags.html
It is because the html file was created from comments in source,
I have to fix the source.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160629234706.31209-1-standby24x7@gmail.com