drm/i915: Move cursor position and base handling into the platform specific functions

Supposedly on some platforms we can get extra atomicity guarantees for
CURPOS if we write it between the CURCNTR and CURBASE. Let's move the
CURPOS handling into the platform specific hooks to make the possible
without having to pass the calculated CURPOS around. And while at it,
do the same for the CURBASE to avoid passing that either.

v2: Use I915_WRITE_FW() and grab uncore.lock

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #v1
Link: http://patchwork.freedesktop.org/patch/msgid/20170327185546.2977-7-ville.syrjala@linux.intel.com