drm/i915: Return stored value from max freq sysfs entry

commit 4f9b2fe0441d4bdf5666a306156b5d6755de2584
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Fri Apr 5 14:29:22 2013 -0700

    drm/i915: Better overclock support

changed the sysfs read semantics for 'gt_max_freq_mhz'. By
always returning overclock max instead of stored value.

Fix this by returning the stored value. Separate sysfs entry
should be considered for overclocking max freq.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=63415


Cc: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>