devfreq: Avoid null pointer de-reference in governor_bw_vbif

Add a null pointer check before calling extern_get_bw().

There is a possibility that devfreq_probe gets called before
kgsl_probe, in such case extern_get_bw() will be null, and will
cause kernel panic.

Change-Id: Icb37e57353fa36c2e8a6dee9357a871a7a795d30
Signed-off-by: Pankaj Gupta <gpankaj@codeaurora.org>