drm/msm/dp: fix error checking for memory allocations

Memory allocation using kzalloc can return a valid pointer
when a zero size memory is requested. Therefore it is
important to use IS_ZERO_OR_NULL macro instead of just
checking for a null pointer prior to any use of the allocated
memory.

CRs-Fixed: 2279573
Change-Id: I3a9526347669d77bd5642195ab2681ef96ce580a
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>