mesa: Add a helper function _mesa_get_min_invocations_per_fragment()

mesa: Add a helper function _mesa_get_min_invocations_per_fragment()

This function is used to test if we need to do per sample shading or
per fragment shading.

V2: Use MAX2() to make sure the function returns a number >= 1.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>