drm/i915/gen9: Sanitize handling of allowed DC states

We can simplify the conditions selecting the target DC state during
runtime by calculating the allowed DC states in advance during driver
loading. This also makes it easier to disable DC states depending on the
i915.disable_power_well module option, added in the next patch.

v2:
- Print a debug message if the requested max DC value was adjusted due
  to a platform limit. Also debug print the calculated mask value. (Patrik)

CC: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456778945-5411-2-git-send-email-imre.deak@intel.com