drm/i915/guc: doorbell checking cleanup

A collection of very small cleanups/improvements around doorbell checking
that do not deserve their own patch:

- Move doorbell-related HW defs to intel_guc_reg.h

- use GUC_NUM_DOORBELLS instead of GUC_DOORBELL_INVALID where
  appropriate

- do not stop on error in guc_verify_doorbells

- do not print drbreg on error: the only content of the register
  apart from the valid bit is the lower part of the physical memory
  address, which we can't use even if valid because we don't know
  which descriptor it came from (since the doorbell is in an unexpected
  state)

- Move the checking of doorbell valid bit to a common helper.

v2: add more cleanups (move defs, use GUC_NUM_DOORBELLS, don't stop in
    guc_verify_doorbells) (Michal)

v3: move more things to intel_guc_reg, redefine
    GUC_DOORBELL_INVALID (Michal), drop guc_doorbell_qw since it just
    duplicates guc_doorbell_info

Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@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/20181022230427.5616-3-daniele.ceraolospurio@intel.com