drm/i915/guc: Make scratch register base and count flexible

We are using some scratch registers in MMIO based send function.
Make their base and count flexible in preparation of upcoming
GuC firmware/hardware changes. While around, change cmd len
parameter verification from WARN_ON to GEM_BUG_ON as we don't
need this all the time.

v2: call out WARN/GEM_BUG change in the commit msg (Daniele)
v3: don't overqualify the ints (Chris)
v4: rebase and use proper enum

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Suggested-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>