firmware: qcom_scm: Permit early SCM calls before driver probe

In some cases we need to make early SCM calls, i.e., when
the driver has not been probed. Adding NULL dereference
checks to handle such cases.

Change-Id: I4e6e6821795ed5833e59350865a17530494c52f9
Signed-off-by: Siddharth Gupta <sidgup@codeaurora.org>
Signed-off-by: Elliot Berman <eberman@codeaurora.org>