video: fbdev: pvr2fb: initialize variables

clang static analysis reports this repesentative error

pvr2fb.c:1049:2: warning: 1st function call argument
  is an uninitialized value [core.CallAndMessage]
        if (*cable_arg)
        ^~~~~~~~~~~~~~~

Problem is that cable_arg depends on the input loop to
set the cable_arg[0].  If it does not, then some random
value from the stack is used.

A similar problem exists for output_arg.

So initialize cable_arg and output_arg.

Signed-off-by: Tom Rix <trix@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200720191845.20115-1-trix@redhat.com