drm/drm_file.c: Define drm_send_event_helper() as 'static'

drm_send_event_helper() has not prototype, it has internal linkage and
therefore it should be defined with storage class 'static'.

Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210427105503.10765-1-fmdefrancesco@gmail.com