drm/i915/gt: replace gen use in intel_engine_cs

Start using the new fields graphics_version for the previous gen checks.
Here we rename the "gen" field and replace the comparisons using it to
start using the new GRAPHICS_VER(). Other uses of INTEL_GEN() were left
as is for automatic conversion later.

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/20210413051002.92589-6-lucas.demarchi@intel.com