drm/i915: fix DP AUX register definitions on Haswell

The old rule that the AUX registers are just an offset (+4 and +10)
from output_reg is not true anymore, since output_reg in on the CPU
and some AUX regs are on the PCH.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: use the existing #defines as spotted by Damien Lespiau.]
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>