drm/amd/display: Pack DMCU iRAM alignment

[Why]
When the DMCU's iRAM definition was moved to the newly created
power_helpers, a #pragma pack was lost, causing the iRAM to be misaligned

[How]
Restore the #pragma pack

Signed-off-by: Josip Pavic <Josip.Pavic@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>