drm/i915/bxt: Clean up bxt_init_clock_gating
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 <90633fea7594af2274c19165794f1103f492dbcc@intel.com>
Cc: Chris Harris <a361e89d1eba6c570561222d75facbbf7aaeeafe@intel.com>
Cc: Jon Bloomfield <e9ad86f33e7e4aee895e71af7518034fb5ddec05@intel.com>
Signed-off-by: Nick Hoath <d8c60c4ef7624f447080543e0ceaab33f523ed33@intel.com>
Reviewed-by: Mika Kuoppala <cd221c71e765d1ce593d15b204fee351fb09fd98@intel.com>
Signed-off-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@ffwll.ch>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs