drm/i915: eliminate remaining uses of intel_device_info->gen

Replace gen with the new graphics_ver value and use GRAPHICS_VER()
in those places.

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/20210413051002.92589-10-lucas.demarchi@intel.com