efi: fix booting failure with UEFI on ARM

efi: fix booting failure with UEFI on ARM

Commit 9fd08b4 (efi: split out efi_get_gop()) splits out the
codes getting the pointer to GOP as efi_get_gop(), but it doesn't
initialize the variable handles and gop to NULL like what the original
codes do. This will cause booting failure on ARM while printing below
logs:
	Xen 4.7-unstable (c/s Tue Oct 13 14:40:28 2015 +0100 git:7a92036) EFI loader
	Synchronous Exception at 0x00000000FECB021C

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>