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

Include <linux/of.h> in source files that need it. Some of DRM's
source code gets OF header via drm_crtc_helper.h and <linux/fb.h>,
which can leed to unnecessary recompilation.

In drm_modes.c, add a comment on the reason for still including
<linux/fb.h>. The header file is required to get KHZ2PICOS(). The
macro is part of the UAPI headers, so it cannot be moved to a less
prominent location.

v2:
	* include <linux/of.h> in komeda_drv.c (kernel test robot)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Liviu Dudau <liviu.dudau@arm.com> # komeda
Acked-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20230111130206.29974-3-tzimmermann@suse.de