drm/i915: Fix setting of boost freq tunable

For limiting the max frequency of gpu, the max freq tunable
is not enough to hard limit the max gap. We now have also per
client boost max freq. When this tunable was introduced,
it was mistakenly made read only. Allow user to gain control by
setting it writable.

Fixes: 29ecd78d3b79 ("drm/i915: Define a separate variable and control for RPS waitboost frequency")
Cc: <stable@vger.kernel.org> # v4.9+
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1481718380-9170-1-git-send-email-mika.kuoppala@intel.com
(cherry picked from commit 73a798711314b54cbd4fe224e24db92c306a8d8c)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>