drm/i915: rename intel_fb.c to intel_fbdev.c

This file is all about the legacy fbdev support. If we want to extract
framebuffer functions, we better put those into a separate file.

Also rename functions accordingly, only two have used the intel_fb_
prefix anyway.

Reviewed-by: Chon Ming Lee <chon.ming.lee@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>