drm/msm/sde: fix feature enable check for CWB support

Fix the Concurrent Writeback feature enabled validation
to check if CWB feature bit set in the Writeback feature flag.

Change-Id: Iffd3167b500edaa3c3338094eadcdc08606ff5c1
Signed-off-by: Prabhanjan Kandula <pkandula@codeaurora.org>