drm/i915: Pull printing GT capabilities on error to err_print_gt

We try not to assume that we captured any information, and so have to
check that error->gt exists before reporting. This check was missed in
err_print_capabilities, so lets break up the capability info and push it
into the GT dump.

We are still a long way from yamlifying this output!

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 792592e7 ("drm/i915: Move the engine mask to intel_gt_info")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200710193239.5419-1-chris@chris-wilson.co.uk