drm/i915/vlv: Use power well CTL IDX instead of ID

Atm, we determine the control/status flag offsets within the PUNIT
control/status registers based on the power well's ID. Since the power
well ID enum is global across all platforms, the associated macros to
get the flag offsets involves some magic. This makes checking the
register/bit definitions against the specification more difficult than
necessary. Also the values in the power well ID enum must stay fixed,
making code maintenance of the enum cumbersome.

To solve the above define the control/status flag indices right after
the corresponding registers and use these to derive the control/status
flag values by storing the indices in the i915_power_well_desc struct.

Initializing anonymous union fields require the preceding field in the
struct to be explicitly initialized - even when using named
initializers - and the initialization to be done right before the union
initialization, hence the reordering of the .id fields.

v2:
- Clarify commit log message about anonymous union initializers. (Paulo)

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180806095843.13294-6-imre.deak@intel.com