drm/i915/selftests: replace unused mask with simple version

Since its introduction 2 years ago, we never used the mask to span more
than one gen. Replace gen_mask a single number and start using the new
GRAPHICS_VER().

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/20210413051002.92589-7-lucas.demarchi@intel.com