drm/i915/selftests: Add a GuC doorbells selftest

drm/i915/selftests: Add a GuC doorbells selftest

The first test aims to check guc_init_doorbell_hw, changing the existing
guc clients and doorbells state before calling it.

The second test tries to create as many clients as it is currently possible
(currently limited to max number of doorbells) and exercise the doorbell
alloc/dealloc code.

Since our usage mode require very few clients/doorbells, this code has
been exercised very lightly and it's good to have a simple test for it.

As reference, this test already helped identify the bug fixed by
commit 908d238a9e78 ("drm/i915/guc: Fix doorbell id selection").

v2: Extend number of clients; check for client allocation failure when
number of doorbells is exceeded; validate client properties; reuse
guc_init_doorbell_hw (Chris).

v3: guc_init_doorbell_hw test added per Chris suggestion.

v4: Try to explain why guc_init_doorbell_hw exist and comment some
details in the subtest.

v5: Remove redundant pr_info at the beginning of each subtest (Chris);
rebase (s/i915_guc_client/intel_guc_client/).

Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171116220632.1909-1-michel.thierry@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>