mesa: add SSBOs to the list of fragment shader side effects

mesa: add SSBOs to the list of fragment shader side effects

The i965 driver uses this function to decide if it can disable the
FS unit in the absence of color/depth writes. We don't want to disable
the unit in the presence of SSBOs, since the fragment shader could
be writing to it.

We could go a step further and check not just for the presence of SSBOs
but also if the shader code writes to them. Does not look worth the trouble
though and we are not doing this for atomic buffers either anyway.

v2: put this into a generic _mesa_active_fragment_shader_has_side_effects
    function instead of having one specific for SSBOs (Jason).

Fixes the following CTS test:
ES31-CTS.shader_storage_buffer_object.advanced-usage-sync-vsfs

Reviewed-by: Francisco Jerez <currojerez@riseup.net>