drm/msm: treat data bus dtsi property as optional

Data bus properties such as number of bus channel and number of
axi ports are optional dtsi properties, and should not cause
data bus parsing function to fail.

Clear error code if the above properties do not exist in dtsi.

CRs-Fixed: 2005348
Change-Id: I44b9d2b462f95444047c8f9f2a83dffae5596074
Signed-off-by: Alan Kwong <akwong@codeaurora.org>