drm/i915/psr: Kill useless function pointers.

At some point we introduced the function pointers
on PSR code to help with VLV/CHV separation logic
because it had a different HW implementation from PSR.

Since all converged to HSW PSR and we dropped the
VLV/CHV support, let's also kill the useless function
pointers and leave the code cleaner.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180626052536.15137-1-rodrigo.vivi@intel.com