drm/i915: finish removal of gen_mask

Now that it's not used anywhere, remove it from struct
intel_device_info. To allow a period in which code will be converted to
the new macro, keep IS_GEN_RANGE() around, just redefining it to use
the new fields. The size advantage from IS_GEN_RANGE() using a mask is
not that big as it has pretty limited use througout the driver:

   text    data     bss     dec     hex filename
2758497   95965    6496 2860958  2ba79e drivers/gpu/drm/i915/i915.ko.old
2758586   95953    6496 2861035  2ba7eb drivers/gpu/drm/i915/i915.ko.new

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/20210413051002.92589-9-lucas.demarchi@intel.com