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.