drm/i915: Remove spurious warning "Failure to install fence"

This particular warning is harmless as we emit during the normal
pinning process where the batch buffer requires more fences than is
available without eviction. Only if we fail to evict enough fences does
this become a problem, so include the requested number of fences in the
ultimate *error* message.

v2: Remember to compile test even trial patches to remove warnings.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>