drm/i915/skl: Enabling PSR on Skylake

Mainly taking care of some register offsets, otherwise things are similar to
hsw. Also, programming ddi aux to use hardcoded values for psr data select.

v2: introduce  EDP_PSR_AUX_BASE macro (Chris)
v3: Moving to HW tracking for SKL+ platforms, so activating source psr during
psr_enabling and then avoiding psr entries and exits for each frontbuffer
updates.
v4: Using SKL DDI AUX regs instead of changing PSR_AUX regs definition (Rodrigo)

Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
[danvet: Drop the hunks to short-circuit sw tracking: We'd need to
push this down one level, and I don't fully trust the test coverage
yet to do so. So much prefer we pick a whitelist approach for the
cases we know work correctly.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>