mesa: Expose compute_component_mapping as _mesa_compute_component_mapping

mesa: Expose compute_component_mapping as _mesa_compute_component_mapping

This is necessary to handle conversions between array types where
the driver does not support the dst format requested by the client and
chooses a different format instead.

We will need this in _mesa_format_convert, so move it to format_utils.c,
prefix it with '_mesa_' and make it available to other files.

v2:
  - Move _mesa_compute_component_mapping to glformats

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>