Add memory operand constraint and write-only modifier to the inline
assembly to effect the writing of the EDID block to boot_params.edid_info.

Without this, gcc would think the EDID query was dead code and would
eliminate it.
Signed-off-by: NAntonino Daplas <adaplas@gmail.com>
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>