drm/amd/pm: Add helper functions for gpu metrics

drm/amd/pm: Add helper functions for gpu metrics

Add helper macros to define metrics struct definitions. It will define
structs with field type followed by actual field. A helper macro is also
added to initialize the field encoding for all fields and to initialize
the field members to 0xFFs.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Asad Kamal <asad.kamal@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>