intel/blorp: Fix usage of uninitialized memory in key hashing

The automatically generated padding in structs contains undefined values. Use

#pragma pack(push, 1)
 ...
#pragma pack(pop)

To eliminate the padding. Otherwise structs with same values may generate different hashes.

Valgrind output:

Conditional jump or move depends on uninitialised value(s)
 util_fast_urem32 (fast_urem_by_const.h:71)
 hash_table_search (hash_table.c:262)
 _mesa_hash_table_search (hash_table.c:296)
 anv_pipeline_cache_search_locked (anv_pipeline_cache.c:318)
 anv_pipeline_cache_search (anv_pipeline_cache.c:335)
 lookup_blorp_shader (anv_blorp.c:38)
 blorp_params_get_mcs_partial_resolve_kernel (blorp_clear.c:1112)
 blorp_mcs_partial_resolve (blorp_clear.c:1205)
 anv_image_mcs_op (anv_blorp.c:1742)
 anv_cmd_predicated_mcs_resolve (genX_cmd_buffer.c:774)
 transition_color_buffer (genX_cmd_buffer.c:1159)
 cmd_buffer_end_subpass (genX_cmd_buffer.c:4840)

Uninitialised value was created by a stack allocation
 blorp_params_get_mcs_partial_resolve_kernel (blorp_clear.c:1103)

I also fixed padding of brw_blorp_const_color_prog_key because clear_rgb_as_red was added to the struct without changing the initial padding (I didn't have valgrind complains about this one).