st/mesa: implement limits for ARB_compute_shader

st/mesa: implement limits for ARB_compute_shader

According to the spec, this also increases the following minimum values:
 - MAX_COMBINED_TEXTURE_IMAGE_UNITS     96 (6*16), was 80
 - MAX_UNIFORM_BUFFER_BINDINGS          72 (6*12), was 60

ARB_compute_shader is not enabled by default because images support is
still not implemented yet. If you want to use it you need to set
MESA_EXTENSION_OVERRIDE=GL_ARB_compute_shader.

Changes from v2:
 - make use of the new PIPE_CAP_SHADER_SUPPORTED_IRS cap instead of
   enabling the extension when PIPE_CAP_COMPUTE is enabled.
 - query for PIPE_CAP_COMPUTE first
 - s/shader_supported_irs/compute_supported_irs/
 - disable ARB_compute_shader and add a comment which explains why

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>