intel/perf: Mark intel_perf_counter_* enums as PACKED

intel/perf: Mark intel_perf_counter_* enums as PACKED

Reduces their sizes from 4 bytes to 1. Cuts 6 KiB from iris_dri.so and
libvulkan_intel.so.

   text    data     bss     dec     hex filename
 924401       0       0  924401   e1af1 meson-generated_.._intel_perf_metrics.c.o (before)
 917613       0       0  917613   e006d meson-generated_.._intel_perf_metrics.c.o (after)

   text    data     bss     dec     hex filename
14137732 365708  210004 14713444 e08264 iris_dri.so (before)
14131044 365708  210004 14706756 e06844 iris_dri.so (after)

   text    data     bss     dec     hex filename
8131009  214264   22820 8368093  7fafdd libvulkan_intel.so (before)
8124321  214264   22820 8361405  7f95bd libvulkan_intel.so (after)

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15237>