llvmpipe: use vpkswss when dst is signed

llvmpipe: use vpkswss when dst is signed

This patch fixes a bug when building a pack instruction.

For POWER (altivec), in case the destination is signed and the
src width is 32, we need to use vpkswss. The original code used vpkuwus,
which emits an unsigned result.

This fixes the following piglit tests on ppc64le:
- spec@arb_color_buffer_float@gl_rgba8-drawpixels
- shaders@glsl-fs-fogscale

I've also corrected some coding style issues in the function.

v2: Returned else statements to vmware style

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>