drm/i915: add new helpers for accessing stepping info

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