drm/i915: rename i915_init_power_well to init_power_domains_init

Similarly rename the other related functions in the power domain
interface.

Higher level driver code calling these functions knows only about power
domains, not the underlying power wells which may be different on
different platforms. Also these functions really init/cleanup/resume
power domains and only through that all related power wells, so rename
them accordingly.

Note that I left i915_{request,release}_power_well as is, since that
really changes the state only of a single power well (and is HSW
specific). It should also get a better name once we make it more
generic by controlling things through a new audio power domain.

v4:
- use intel prefix instead of i915 everywhere (Paulo)
- use a $prefix_$block_$action format (Daniel)

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