Add new runtime info field for stepping. Add new helpers for accessing
them. As we'll be switching platforms over to the new scheme
incrementally, check for non-initialized steppings.

In case a platform does not have separate display and gt steppings, it's
okay to use a common shorthand. However, in this case the display
stepping must not be initialized, and gt stepping is the single point of
truth.

v3: Remove IS_STEP() (José)

v2: Rename stepping->step
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/bb4275733fa390ea3dbf6f62794d55b616665230.1616764798.git.jani.nikula@intel.com