drm/i915: HSW PM Frequency bits fix

According to HSW PM programming guide, frequency bits starts at
24 instead of 25.

v2: Paulo Zanoni noticed that only frequency bits can be set at
GEN6_RPNSWREQ. All others are read only.

CC: Ben Widawsky <ben@bwidawsk.net>
CC: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>