drm/i915: Make AGP=n work even on gen3

Most platforms din't hit this condition, but if we want to allow
building without agp we should also make this allowed on gen3.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.c

@@ -158,7 +158,7 @@ static struct drm_driver driver;
 #if IS_ENABLED(CONFIG_AGP_INTEL)
 extern int intel_agp_enabled;
 #else
-static int intel_agp_enabled;
+static int intel_agp_enabled = 1;
 #endif
 
 static const struct intel_device_info intel_i830_info = {