drm/i915: finish removal of gen from intel_device_info

Now that it's not being used anymore, finish its removal. Like for
gen_mask, we replace INTEL_GEN() and IS_GEN()  macros to use the new
field.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
[Jani: Minor code comment change while applying.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210413051002.92589-11-lucas.demarchi@intel.com