radv: Don't compile shaders when they are cached already.

radv: Don't compile shaders when they are cached already.

When the gs_copy_shader is NULL (due to an incomplete cache), but
the main shaders are found, we still do the nir, but we shouldn't
compile the shaders again. For merged shaders we should also account
for the missing shaders.

Fixes: ce03c119ce0 'radv: Add code to compile merged shaders.'
Reviewed-by: Dave Airlie <airlied@redhat.com>