drm/amd/pm: avoid unaligned access warnings

When building on OpenBSD/arm64 with clang 15, unaligned access
warnings are seen when a union is embedded inside a packed struct.

drm/amd/pm/powerplay/hwmgr/vega20_pptable.h:136:17: error: field
  smcPPTable within 'struct _ATOM_VEGA20_POWERPLAYTABLE' is less aligned
  than 'PPTable_t' and is usually due to
  'struct _ATOM_VEGA20_POWERPLAYTABLE' being packed, which can lead to
   unaligned accesses [-Werror,-Wunaligned-access]
      PPTable_t smcPPTable;
                ^

Make PPTable_t packed to avoid this.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>