dispatch: Add function piglit_dispatch_get_api

The new function returns the API to which dispatch was earlier
initialized, and may return PIGLIT_DISPATCH_UNINITIALIZED.

This patch also adds the convenience function piglit_dispatch_is_init,
which simply checks if the current api is PIGLIT_DISPATCH_UNINITIALIZED.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
WIP util: Merge piglit_num_components()
WIP util: Merge piglit_get_luminance_intensity_bits()
WIP util: Merge piglit_probe_pixel_rgba()