gallium: increase pipe_sampler_view::target bitfield size for MSVC

gallium: increase pipe_sampler_view::target bitfield size for MSVC

MSVC treats enums as being signed.  The 4-bit target field isn't large
enough to correctly store the value 8 (for PIPE_TEXTURE_CUBE_ARRAY).
The bitfield value 0x8 was being interpreted as -8 so matching the
target with PIPE_TEXTURE_CUBE_ARRAY in switch statements, etc. was
failing.

To keep the structure size the same, we reduce the format field from
16 bits to 15.  There don't appear to be any other enum bitfields
which need to be adjusted.

This fixes a number of Piglit cube map array tests.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>