mesa: Add a _mesa_active_fragment_shader_has_side_effects helper

mesa: Add a _mesa_active_fragment_shader_has_side_effects helper

Some drivers can disable the FS unit if there is nothing in the shader code
that writes to an output (i.e. color, depth, etc). Right now, mesa has
a function to check for atomic buffers and the i965 driver also checks for
images. Refactor this logic into a generic function that we can use for
any source of side effects in a fragment shader. Suggested by Jason.

v2:
- Use '_Shader', as suggested by Tapani, to fix the following CTS test:

ES31-CTS.shader_atomic_counters.advanced-usage-many-draw-calls2

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