i915: fix compiler warning

The last i915 drm update brought with it this annoying warning

  drivers/gpu/drm/i915/intel_crt.c: In function ‘intel_crt_get_config’:
  drivers/gpu/drm/i915/intel_crt.c:110:21: warning: unused variable ‘dev’ [-Wunused-variable]
    struct drm_device *dev = encoder->base.dev;
                       ^

introduced by commit 7195a50b5c7e ("drm/i915: Add HSW CRT output readout
support").

Remove the offending pointless variable.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>