drm/i915/dmc: Eliminate remnant GEN<n> references

Replace GEN<n> with DISPLAY_VER<n>, in line with the naming convention
followed in the i915 driver code.

Signed-off-by: Madhumitha Tolakanahalli Pradeep <madhumitha.tolakanahalli.pradeep@intel.com>
Reviewed-by: Caz Yokoyama <caz.yokoyama@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211217034141.198033-1-madhumitha.tolakanahalli.pradeep@intel.com