efi/gop: Add an option to list out the available GOP modes

Add video=efifb:list option to list the modes that are available.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200518190716.751506-20-nivedita@alum.mit.edu


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