drm/i915/csr: bypass firmware request on i915.dmc_firmware_path=""

With i915.dmc_firmware_path="" it's obvious the intention is to disable
CSR firmware loading. Bypass the firmware request altogether in this
case, with more obvious debug logging.

v2: Use DRM_INFO for logging (Chris)
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180926133414.22073-3-jani.nikula@intel.com

drivers/gpu/drm/i915/intel_csr.c

@@ -485,6 +485,12 @@ void intel_csr_ucode_init(struct drm_i915_private *dev_priv)
 	}
 
 	if (i915_modparams.dmc_firmware_path) {
+		if (strlen(i915_modparams.dmc_firmware_path) == 0) {
+			csr->fw_path = NULL;
+			DRM_INFO("Disabling CSR firmare and runtime PM\n");
+			return;
+		}
+
 		csr->fw_path = i915_modparams.dmc_firmware_path;
 		/* Bypass version check for firmware override. */
 		csr->required_version = 0;