drm/i915/icl: Cleanup combo PHY aux power well handlers

Now that the combo PHY aux power well handlers are used exclusively on
Icelake, we can drop a bunch of the extra tests.

v2: Don't try to use intel_uncore_rmw for register updates yet; there's
    pending display uncore patches that need to land first.  (Lucas)

v3: Drop the combo phy assertion.  It was backward before, but doesn't
    seem terribly necessary.  I'm keeping the IS_ICELAKE assertion
    though since we often copy/paste/modify the power well tables when
    defining new platforms and it's too easy to cargo cult the
    ICL-specific handling to new platforms that shouldn't use it.
    (Lucas)

v4: Fix build; forgot to commit all the changes.  (CI)

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191213010600.701315-1-matthew.d.roper@intel.com
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>