efi/gop: Use helper macros for populating lfb_base

Use the lower/upper_32_bits macros from kernel.h to initialize
si->lfb_base and si->ext_lfb_base.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200320020028.1936003-8-nivedita@alum.mit.edu


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>