efi/gop: Allow specifying depth as well as resolution

Extend the video mode argument to handle an optional color depth
specification of the form
	video=efifb:<xres>x<yres>[-(rgb|bgr|<bpp>)]

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200320020028.1936003-14-nivedita@alum.mit.edu


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