st/dri: check pscreen is valid before querying param
st/dri: check pscreen is valid before querying param

we don't check the validity of pscreen until dri_init_screen_helper

hit this trying to init glamor on a device with no driver (udl).

Acked-by: Michel Dänzer <242b79f0804cf52679d58f27ed754158c1323c75@amd.com>
Signed-off-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler