drm/i915: Fix handling of non-supported uC

drm/i915: Fix handling of non-supported uC

There are 2 issues around handling of missing uC support:

- We treat lack of uC HW and lack of uC FW definition as 2 different
  cases, but both of them mean that we don't support the uC on the
  platform we're running on.

- We rely on the modparam to decide if we can take uC paths or not, but
  we don't sanitize it if it is set incorrectly on platform with no uC
  support.

To fix both of them, unify the 2 cases in a single one and sanitize the
modparam on invalid configuration (after printing an error message).
The log has been adapted as well, since the user doesn't care why we
don't support GuC/HuC (no HW or no FW), just that we do not. Developers
can easily find the answer based on the platform, so we can simplify the
log.

Correcting the modparam has been preferred over failing the load since
this is what we usually do for non-supported feature (e.g. the now gone
enable_ppgtt would fall back to the highest supported PPGTT mode if the
selected one was not available).

Note that this patch purposely doesn't change the behavior for platforms
that do have uC support, in which case we will still fail if enable_guc
is set and the firmware is not available on the system.

Suggested-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190725001813.4740-3-daniele.ceraolospurio@intel.com