isl: Mark enum isl_channel_select packed so it becomes 1 byte.

isl: Mark enum isl_channel_select packed so it becomes 1 byte.

I recently discovered that the following code lead to valgrind errors:

   struct isl_swizzle swizzle = ISL_SWIZZLE_IDENTITY;
   VALGRIND_CHECK_MEM_IS_DEFINED(&swizzle, sizeof(swizzle));

which is surprising, because struct isl_swizzle is simply:

   struct isl_swizzle {
      enum isl_channel_select r:4;
      enum isl_channel_select g:4;
      enum isl_channel_select b:4;
      enum isl_channel_select a:4;
   };

and the above code initializes all of them with a C99 initializer.
Iván Briano reminded me that C99 initializers don't necessarily zero
padding.  A quick inspection revealed that sizeof(struct isl_swizzle)
was 4 (rather than the expected 2).  Ian Romanick suggested changing
it to uint16_t, since this is essentially dicing up an unsigned, and
that worked.

This patch marks enum isl_channel_select packed, changing its size
from 4 bytes to 1 byte.  This then makes struct isl_swizzle 2 bytes,
with no bogus padding fields.  This eliminates valgrind undefined
memory warnings.

These isl_swizzle values become part of our BLORP blit program keys,
which are then hashed.  This undefined padding was being included in
the hashing, possibly leading to issues.  I originally saw this error
when running KHR-GL45.texture_size_promotion.functional in iris under
valgrind.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>