drm: don't set the signal blocker on the master process.

There is a problem with debugging the X server and gdb crashes in
the xkb startup code.

This avoids the problem by allowing the master process to get signals.
It should be safe as the signal blocker is mainly so that you can
Ctrl-Z a 3D application without locking up the whole box. Ctrl-Z the
X server isn't something many people do.

Signed-off-by: Dave Airlie <airlied@redhat.com>