efi/gop: Allow specifying mode by <xres>x<yres>

Add the ability to choose a video mode using a command-line argument of
the form
	video=efifb:<xres>x<yres>

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200320020028.1936003-13-nivedita@alum.mit.edu


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