drm/i915: Factor out intel_tile_width()

Pull the tile width calculations from intel_fb_stride_alignment() into a
new function intel_tile_width().

Also take the opportunity to pass aroun dev_priv instead of dev to
intel_fb_stride_alignment().

v2: Reorder argumnents to be more consistent with other functions
    Change intel_fb_stride_alignment() to accept dev_priv instead of dev
v3: Deal with Y tilling (Daniel)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1452625717-9713-3-git-send-email-ville.syrjala@linux.intel.com