drm/i915: add a default always-on power well

So far we distinguished platforms without a dynamic power well with
the HAS_POWER_WELL macro and for such platforms we didn't call any power
domain functions. Instead of doing this check we can add an always-on
power well for these platforms and call the power domain functions
unconditionally. For always-on power wells we only increase/decrease
their refcounts, otherwise they are nop.

This makes high level driver code more readable and as a bonus provides
some idea of the current power domains state for all platforms (once
the relevant debugfs entry is added).

v3: rename intel_power_wells to i9xx_always_on_power_well (Paulo)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>