drm/i915/skl: vfuncs for skl eld and global resource

Set gen 9 function pointers for eld write and global resource.
Implementation remains same as HSW.

v2: Rebase on top of Sonika's untangling of the if/else ladder (Damien)

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Satheeshakrishna M <satheeshakrishna.m@intel.com> (v1)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>