drm/i915/bios: Downgrade the "signature missing" DRM_ERROR to debug

As we warn the user later that we cannot find or load the VBIOS,
explaining why is an exercise in debugging. Shouting *ERROR* upsets
people and produces bug reports.

Reported-by: Michael Rieder <mr@student.ethz.ch>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=43751
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_bios.c

@@ -669,7 +669,7 @@ intel_parse_bios(struct drm_device *dev)
 		}
 
 		if (!vbt) {
-			DRM_ERROR("VBT signature missing\n");
+			DRM_DEBUG_DRIVER("VBT signature missing\n");
 			pci_unmap_rom(pdev, bios);
 			return -1;
 		}