drm/i915: Downgrade unknown CSR firmware warnings

Like it was done in commit 9e180d9991dc ("drm/i915: Downgrade unknown
firmware warnings") for huc and guc: downgrade CSR firmware warnings. If
we have released no firmware yet for a platform, stop scaring the
consumer and merely note its expected absence.

By simply removing the warning and early return we hit the condition
with the appropriate message.

Cc: Chris Wilson <[e-mail]>
Cc: Joonas Lahtinen <[e-mail]>
Signed-off-by: Lucas De Marchi <[e-mail]>
Reviewed-by: Chris Wilson <[e-mail]>
Reviewed-by: Rodrigo Vivi <[e-mail]>
Signed-off-by: Rodrigo Vivi <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/intel_csr.c

···
 		csr->fw_path = BXT_CSR_PATH;
 		csr->required_version = BXT_CSR_VERSION_REQUIRED;
 		csr->max_fw_size = BXT_CSR_MAX_FW_SIZE;
-	} else {
-		MISSING_CASE(INTEL_REVID(dev_priv));
-		return;
 	}
 
 	if (i915_modparams.dmc_firmware_path) {