drm/i915: Explain usage of power well IDs vs bit groups

v2: Add explanation of the fixed power well bits (Imre)

Signed-off-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447682467-6237-2-git-send-email-patrik.jakobsson@linux.intel.com