i965: Add an option to ignore sample qualifier

i965: Add an option to ignore sample qualifier

This will be useful in my next patch which depends on a functionality
of _mesa_get_min_invocations_per_fragment() to ignore the sample
qualifier (prog->IsSample) based on a flag passed to it.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>