Use write-combining MTRR to speed up video with buggy BIOSes.

	Use write-combining MTRR to speed up video with buggy BIOSes.

	* grub-core/video/i386/pc/vbe.c (framebuffer): New member mtrr.
	(cpuid): New define.
	(rdmsr): Likewise.
	(wrmsr): Likewise.
	(mtrr_base): Likewise.
	(mtrr_mask): Likewise.
	(grub_vbe_enable_mtrr_entry): New function.
	(grub_vbe_enable_mtrr): Likewise.
	(grub_vbe_disable_mtrr): Likewise.
	(grub_vbe_bios_set_display_start): Disable mtrr when handing the
	control off to BIOS.
	(grub_video_vbe_init): Fill mtrr.
	(grub_video_vbe_fini): Disable mtrr.
	(grub_video_vbe_get_info_and_fini): Likewise.
	(grub_video_vbe_setup): Enable mtrr.