drm/i915/fb: move user framebuffer stuff to intel_fb.c

Split out fb related stuff from intel_display.c to intel_fb.c.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/76b61738857619c1cce6e4306d14da19ee3bbf08.1629721467.git.jani.nikula@intel.com