drm/i915: Remove .is_mobile field from platform struct

As recommended by Ville Syrjala removing .is_mobile field from the
platform struct definition for vlv and hsw+ GPUs as there's no need to
make the distinction in later hardware anymore. Keep it for older GPUs
as it is still needed for ilk-ivb.

Signed-off-by: Carlos Santa <carlos.santa@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>