drm/msm/sde: update fast clear enable logic

If blending is disabled from client through blend
property on plane, avoid enabling UBWC fast clear
bit for sspp. UBWC fast clear is applicable only
if blending needs to be enabled on a plane.

Change-Id: I5d2cdd4e5d90e9c584e9cd9854e6ac19e11c3c12
Signed-off-by: Prabhanjan Kandula <pkandula@codeaurora.org>