drm/i915: Removed the read of RP_STATE_CAP from sysfs/debugfs functions

The frequency values(Rp0, Rp1, Rpn) reported by RP_STATE_CAP register
are stored, initially by the Driver, inside the dev_priv->rps structure.
Since these values are expected to remain same throughout, there is no real
need to read this register, on dynamic basis, from certain debugfs/sysfs
functions and the values can be instead retrieved from the dev_priv->rps
structure when needed.
For the i915_frequency_info debugfs interface, the frequency values from the
RP_STATE_CAP register only should be used, to indicate the actual Hw state,
since it is principally used for the debugging purpose.

v2: Reverted the changes in i915_frequency_info function, to continue report
    back the frequency values, as per the actual Hw state (Chris)

Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>