drm/i915: Remove the magic AUX_CTL is at DP + foo tricks

Currently we determine the location of the AUX registers in a confusing
way. First we assume the PCH registers are used always, but then we
override it for everything but HSW/BDW to use DP+0x10. Very confusing.

Let's just make it straightforward and simply add a few functions to
pick the right AUX_CTL based on the DP port.

To deal with VLV/CHV we'll include the display_mmio_offset into the
AUX register defines.

v2: Reorder patches (Chris)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v1)
Link: http://patchwork.freedesktop.org/patch/msgid/1447266856-30249-5-git-send-email-ville.syrjala@linux.intel.com