drm/i915: Remove unused i915_err_print_instdone

Just a simple code cleanup, below commit forgot to remove a
function which it made unused:

 commit eaa14c24
 Author: Chris Wilson <chris@chris-wilson.co.uk>
 Date:   Wed Oct 19 13:52:03 2016 +0100

  drm/i915: Stop reporting error details in dmesg as well as the error-state

  As we already capture all the information from the registers into the
  error-state, also dumping that to dmesg just generates noise that upsets
  CI and users alike (and doesn't provide us with any more information).

v2: Chris Wilson dag out the relevant commit. Commit msg updated.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170727110113.16942-1-tvrtko.ursulin@linux.intel.com