drm/i915: kill intel_resume_prepare()

Because, really, the abstraction is not working for us. It is nice for
VLV, but doesn't add anything useful on SNB/HSW/BDW. We want to change
this code due to a recently-discovered bug, but we can't seem to find
a nice solution that repects the current abstraction. So let's kill
intel_resume_prepare() and its friends, and add an equivalent
implementation to both its callers.

Also, look at the diffstat!

v2: - Rebase.

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