drm/i915: Fix PM refcounting w/o DMC firmware

The case where the firmware isn't specified for a platform (although
runtime PM works only with DMC on this platform) is the same case where
the firmware is specified but can't be loaded for some reason. Hence we
need to get a display init power domain ref in the first case too to
keep the refcount bookkeeping in balance.

Also convert the related log message to be a debug one, since it's a
valid scenario for a new platform, where we need to have
dev_info->has_csr=1 set, but add support for actually loading the
firmware only later.

v2:
- In addition to the debug log, WARN on non-alpha support platforms,
  since then the first case isn't valid scenario. (Chris)

References: https://bugs.freedesktop.org/show_bug.cgi?id=107382


Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180815131038.24446-1-imre.deak@intel.com