drm/i915: Improve execbuf debug

Convert i915_gem_check_execbuffer to return the error code instead of
a boolean so our neat EINVAL debugging trick works within this function.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191209122314.16289-1-tvrtko.ursulin@linux.intel.com