gallium: Add a cap bit for integer multiplication between 32-bit and 16-bit

gallium: Add a cap bit for integer multiplication between 32-bit and 16-bit

Driver supports integer multiplication between a 32-bit integer and a
16-bit integer.  If the second operand is 32-bits, the upper 16-bits are
ignored, and the low 16-bits are possibly sign extended as necessary.

Iris will eventually enable this.  Not sure about other drivers.

v2: Add default value to u_screen.c.  Suggested by Caio.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/767>