drm/i915: Add sprite watermark programming for VLV and CHV

Program DDL register as part of sprite watermark programming for CHV and VLV.

v2: Rename DRAIN_LATENCY_MAX by DRAIN_LATENCY_MASK

v3: Addressed review comments by Ville
    - Changed Sprite DDL definitions to more generic to avoid multiple if-else
    - Changed bit masking to customary form
    - Changed to bitwise shorthand operator for sprite_dl assignment

Signed-off-by: Gajanan Bhat <gajanan.bhat@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>