drm/i915: vlv: clamp minimum RPS frequency to what Punit allows

As described in the code comment, I couldn't set the minimum RPS
frequency on my BYT-M B0 to the minimum allowed as reported by Punit.
Fix this by clamping the minimum value to the first one that was
accepted on my machine.

Atm this issue doesn't cause any test failures, since in igt/pm_rps we
only check the cached version of the current frequency which is the same
what has been set. In the future we'll add checks for the actual
frequency too, and for that to pass this fix is necessary.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1417711175-13271-1-git-send-email-imre.deak@intel.com