drm/i915: Catch abuse of I915_EXEC_CONSTANTS_*

A bit tricky since 0 is also a valid constant ...

v2: Add DRM_DEBUG (Chris)

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

diff --cc drivers/gpu/drm/i915/i915_gem_execbuffer.c
Simple merge