drm/i915/gt: Only unwedge if we can reset first

Unwedging the GPU requires a successful GPU reset before we restore the
default submission, or else we may see residual context switch events
that we were not expecting.

v2: Pull in the special-case reset_clobbers_display, and explain why it
should be safe in the context of unwedging.

v3: Just forget all about resets before unwedging if it will clobber the
display; risk it all.

Reported-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20190927160335.10622-1-chris@chris-wilson.co.uk