drm/i915/display: split i915 specific code from intel_fbdev

Split out code from intel_fbdev that can not be share between i915 and
xe. Create new i915 specific source/header file intel_fbdev_fb.[ch] which
contains this code.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231115090719.3210079-2-jouni.hogander@intel.com