drm/i915: Catch abuse of I915_EXEC_GEN7_SOL_RESET

Currently we catch it, but silently succeed. Our userspace is
better than this.

v2: Add DRM_DEBUG (Chris)

Testcase: igt/gem_exec_params/sol-reset-*
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>