drm/i915/bxt: Clean up bxt_init_clock_gating

Add stepping check for A0 workarounds, and remove the associated
FIXME tags.
Split out unrelated WAs for later condition checking.

v2: Fixed format (PeterL)
v3: Corrected stepping check for WaDisableSDEUnitClockGating
    - Ignoring comment, following hardware spec instead. (ChrisH)
    Added description for TILECTL setting (JonB)

Cc: Peter Lawthers <peter.lawthers@intel.com>
Cc: Chris Harris <chris.harris@intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>