drm/msm/dpu: fix for cursor blend issue

drm/msm/dpu: fix for cursor blend issue

The current driver has the opaque blend mode set as the
default causing the black box effect around the cursor.
The fix enables choosing a different blend mode for alpha
enabled formats.

Changes in V2:
	- Use drm_get_format_name() in the logs (Sean)

Signed-off-by: Sravanthi Kollukuduru <skolluku@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>