[x86 setup] video setup: Fix VBE DDC reading

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: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>