x86/umip: Factor out instruction decoding

Factor out the code used to decode an instruction with the correct
address and operand sizes to a helper function.

No functional changes.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200907131613.12703-10-joro@8bytes.org