efifb: Omit memory map check on legacy boot

Since the following commit:

  38ac0287 ("fbdev/efifb: Honour UEFI memory map attributes when mapping the FB")

efifb_probe() checks its memory range via efi_mem_desc_lookup(),
and this leads to a spurious error message:

   EFI_MEMMAP is not enabled

at every boot on KVM.  This is quite annoying since the error message
appears even if you set "quiet" boot option.

Since this happens on legacy boot, which strangely enough exposes
a EFI framebuffer via screen_info, let's double check that we are
doing an EFI boot before attempting to access the EFI memory map.

Reported-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Jones <pjones@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20190328193429.21373-3-ard.biesheuvel@linaro.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>