drm/i915/skl: Read the Memory Latency Values for WM computation

This patch reads the memory latency values for all the 8 levels for
SKL. These values are needed for the Watermark computation.

v2: Incorporated the review comments from Damien on register
    indentation.

v3: Updated the code to use the sandybridge_pcode_read for reading
    memory latencies for GEN9.

v4: Don't put gen 9 in the middle of an ordered list of ifs
    (Damien)

v5: take the rps.hw_lock around sandybridge_pcode_read() (Damien)

v6: Use gen >= 9 in the pcode_read() function for data1.
    Move the defines near the gen6 ones and prefix them with PCODE.
    Remove unused timeout define (the pcode_read() code has a larger
    timeout already).

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Pradeep Bhat <pradeep.bhat@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>