drm/i915/dmc: Change intel_get_stepping_info()

Lets use RUNTIME_INFO->step since all platforms now have their
stepping info in intel_step.c. This makes intel_get_stepping_info()
a lot simpler.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210721215238.24980-2-anusha.srivatsa@intel.com