drm/i915/skl: Retrieve the Rpe value from Pcode

Read the efficient frequency (aka RPe) value through the the mailbox
command (0x1A) from the pcode, as done on Haswell and Broadwell.
The turbo minimum frequency softlimit is not revised as per the
efficient frequency value.

v2: Replaced the conditional expression operator with 'if' statement (Tom)
v3: Corrected the derivation of efficient frequency & shifted the
    GEN9_FREQ_SCALER multiplications downwards (Ville)

Issue: VIZ-5143
Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>