drm/i915/psr: Rewrite comments in intel_psr_wait_for_idle()

Added bspec reference, aligned text and documented the function.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180824230844.12428-2-dhinakaran.pandiyan@intel.com