drm/i915/selftests: eliminate use of gen_mask

Remove the remaining uses of INTEL_GEN_MASK() and the correspondent
gen_mask in struct intel_device_info. This will allow the removal of
gen_mask later since it's incompatible with the new per-IP versioning
scheme.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210413051002.92589-8-lucas.demarchi@intel.com