drm/i915/bxt, glk: Fix assert on conditions for DC9 enabling

What we want to assert based on the conditions required by Bspec is that
power well 2 is disabled, so no need to check for other power wells.
In addition we can only check if the driver's request is removed, the
actual state depends on whether the other request bits are set or not
(BIOS, KVMR, DEBUG). So check only the driver's request bit.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1498750622-14023-4-git-send-email-imre.deak@intel.com