drm/i915: Downgrade unknown CSR firmware warnings

Like it was done in commit 9e180d99 ("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 <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181117004234.23437-2-lucas.demarchi@intel.com