intel/isl: Add an isl_swizzle structure and use it for isl_view swizzles

intel/isl: Add an isl_swizzle structure and use it for isl_view swizzles

This should be more compact than the enum isl_channel_select[4] that we
were using before.  It's also very convenient because we already had such a
structure in the Vulkan driver we just needed to pull it over.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>