drm/i915/skl: Register definitions and macros for SKL Watermark regs

This patch defines SKL specific PLANE_WM Watermark registers. It also
defines macros to get the addresses of different LP levels within a pipe.

v2: Reworked the register definitions and associated macros to make it more
    generic and be able to use for_each_pipe in values computation.
    Incorporated Damien's review comments and indentation.

v3: Added default values for lines and blocks. Provided mask for blocks.

v4: Prefix intermedidate (internal-only) macros with _ (Ville)

v5: Remove the lines and block defaults value (Ville)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v4)
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>