drm/i915/vlv: Remove redundant power well ID asserts

The callbacks these asserts are called from are used from a single power
well, so not much point in checking that. The check also requires a unique
power well ID that we would need to keep around only for this purpose.
(A follow-up patch removes power well IDs not needed for direct power
 well access).

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180806095843.13294-4-imre.deak@intel.com