drm/i915/uc: Unify uc_fw status tracking

We currently track fetch and load status separately, but the 2 are
actually sequential in the uc lifetime (fetch must complete before we
can attempt the load!). Unifying the 2 variables we can better follow
the sequential states and improve our trackng of the uC state.

Also, sprinkle some GEM_BUG_ON to make sure we transition correctly
between states.

v2: rename states, add the running state (Michal), drop some logs in
    the fetch path (Michal, Chris)

v3: re-rename states, extend early status check to all helpers (Michal)

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>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
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/20190725001813.4740-5-daniele.ceraolospurio@intel.com