drm: Correct a typo in a function comment

Replace "pionter" with "pointer" in the drm_gem_handle_create description.

Changes in v2:
- Change subject text

Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200317210339.2669-1-igormtorrente@gmail.com