r600: work out shader export mask at shader build time (v1.1)

r600: work out shader export mask at shader build time (v1.1)

Since enhanced layouts allows setting specific MRT outputs, we
can get sparse outputs, so we have to calculate the shader
mask earlier.

v1.1: update checks for state update (Roland)

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>