efi/gop: Factor out locating the gop into a function

Move the loop to find a gop into its own function.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200320020028.1936003-5-nivedita@alum.mit.edu


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