drm: Include <linux/backlight.h> where needed

Include <linux/backlight.h> in source files that need it. Some of
DRM's source code gets the backlight header via drm_crtc_helper.h
and <linux/fb.h>, which can leed to unnecessary recompilation. If
possible, do not include drm_crtc_helper.h any longer.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Christian König <christian.koenig@amd.com> # amd
Acked-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20230111130206.29974-2-tzimmermann@suse.de