qseecom: Check error when allocating coherent buffer

Add check to verify whether memory is allocated properly or not
and return error if its failed to allocate coherent buffer.

Change-Id: I234a637d20228d047f6fc11d374b6f13f28fed03
Signed-off-by: Gaurav Kashyap <gaurkash@codeaurora.org>
Signed-off-by: AnilKumar Chimata <anilc@codeaurora.org>