drm/radeon/kms: add MSI module parameter

Allow the user to override whether MSIs are enabled
or not on supported ASICs.  MSIs are disabled by default
on IGP chips as they tend not to work.  However certain
IGP chips only seem to work with MSIs enabled.

I suspect this is a chipset or bios issue, but I'm not sure
what the proper fix is.  This will at least make diagnosing
and working around the problem much easier.

See:
https://bugs.freedesktop.org/show_bug.cgi?id=37679



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>