drm/i915: Add fault injection support

Add support for forcing an error at selected places in the driver. As an
example add 4 options to fail during driver loading.

Requested by Chris.

v2:
- Add fault point for modeset initialization
- Print debug message when injecting an error
v3:
- Rename inject_fault to inject_load_failure, rename the related macros
  and helper accordingly (Chris)
- Use a counter instead of a mask to identify the failure point (Daniel)
- Mark the module option as _unsafe and keep i915_params ordered (Joonas)
v4:
- Rebase on latest -nightly
v5:
- Use DRM_INFO instead of DRM_DEBUG_DRIVER, making it clearer in CI reports
  that a following error message is expected (IRC r-b from Chris on v5)

CC: Chris Wilson <chris@chris-wilson.co.uk>
CC: Daniel Vetter <daniel.vetter@ffwll.ch>
CC: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>