drm/i915: Don't pass pitch to intel_compute_page_offset()

intel_compute_page_offset() can dig up the correct pitch from the fb
itself, no need for the caller to pass it in.

A bit of extra care is needed for the lower level
_intel_compute_page_offset() since that one gets called before the
rotated pitch under intel_fb is populated. Note that we don't actually
call it with anything but DRM_ROTATE_0 there so we wouldn't actually
look up the rotated pitch there, but still, leave the pitch as something
the caller has to pass to _intel_compute_page_offset() as an
indicator that something is a bit special.

This leaves 'stride_div' in the skl plane update hooks as a mostly useless
variable so just get rid of it.

v2: Add a note why stride_div got nuked
v3: Extract intel_fb_pitch() since it can be useful later

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v2)
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470821001-25272-3-git-send-email-ville.syrjala@linux.intel.com