drm/i915/guc: Don't read SOFT_SCRATCH(15) on MMIO error

SOFT_SCRATCH(15) is used by GuC for sending MMIO GuC events to host and
those events are now handled by intel_guc_to_host_event_handler_mmio().

We should not try to read it on MMIO action error as 1) we may be using
different set of registers for GuC MMIO communication, and 2) GuC may
use CTB mechanism for sending events to host.

While here, upgrade error message to DRM_ERROR.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180528171618.10436-1-michal.wajdeczko@intel.com