drm: Spelling fixes

Minor spelling fix for 'monster' and replace 'on' with 'own' in
comments.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171101140436.2743-1-Liviu.Dudau@arm.com