drm/i915: Merge sbi read/write into a single accessor

Since intel_sideband_read and intel_sideband_write differ by only a
couple of lines (depending on whether we feed the value in or out),
merge the two into a single common accessor.

v2: Restore vlv_flisdsi_read() lost during rebasing.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190426081725.31217-6-chris@chris-wilson.co.uk