drm/i915: Add is_dgfx to device info

This will be helpful to diferentiate a set of GPUs
with the same GEN version.

Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191024195122.22877-1-lucas.demarchi@intel.com