drm/i915: fold in __intel_power_well_get/put functions

These functions are used only by a single call site and are simple
enough to just fold them in.

Note that in later patches the parts folded in here are further
simplified as we'll remove hsw_{disable,enable}_package_c8 and the NULL
check of the power well enable/disable handlers. All this means that at
the end intel_display_power_get/put() becomes more understandable as we
don't need to jump between two functions when reading the code.

No functional change.

v2:
- clarify the rational for the change (Chris)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>