msm: mdss: correct the validating method of debug name string

The name member in mdss_debug_base struct is not a pointer,
so the current method to check whether this name string is
empty is invalid. Use strlen to fix this bug which might
introduce NULL pointer references in some cases if the name
string is empty.

Change-Id: I52b5eeae5f303e5690ce1e6a4ce00debf3435ee2
Signed-off-by: Ray Zhang <rayz@codeaurora.org>