drm/i915: Generalize drain latency computation

Modify drain latency computation to use it for any plane. Same function can be
used for primary, cursor and sprite planes.

v2: Adressed review comments by Imre and Ville.
    - Moved clock round up in separate patch
    - Added WARN check for clock and pixel size
    - Simplified bit masking
    - Use cursor_base instead of reg read

v3: Changed to bitwise shorthand operator for plane_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>