drm/i915: Move fbdev functions

drm/i915: Move fbdev functions

Move functions within intel_fbdev.c to simplify later updates. Minor
style fixes to make checkpatch happy, but no functional changes.

v5:
- style fixes (checkpatch)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240409081029.17843-3-tzimmermann@suse.de
Signed-off-by: Jani Nikula <jani.nikula@intel.com>