drm/i915: fix intel_init_power_wells

drm/i915: fix intel_init_power_wells

The current code was wrong in many different ways, so this is a full
rewrite. We don't have "different power wells for different parts of
the GPU", we have a single power well, but we have multiple registers
that can be used to request enabling/disabling the power well. So
let's be a good citizen and only use the register we're suppose to
use, except when we're loading the driver, where we clear the request
made by the BIOS.

If any of the registers is requesting the power well to be enabled, it
will be enabled. If none of the registers is requesting the power well
to be enabled, it will be disabled.

For now we're just forcing the power well to be enabled, but in the
next commits we'll change this.

V2:
  - Remove debug messages that could be misleading due to possible
    race conditions with KVMr, Debug and BIOS.
  - Don't wait on disabling: after a conversaion with a hardware
    engineer we discovered that the "restriction" on bit 31 is just
    for the "enable" case, and we don't even need to wait on the
    "disable" case.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>