[media] vb2: fix compiler warning

[media] vb2: fix compiler warning

When compiling this for older kernels using the compatibility build
the compiler complains about uninitialized variables:

In file included from include/linux/kernel.h:20:0,
                 from include/linux/cache.h:4,
                 from include/linux/time.h:7,
                 from include/linux/input.h:13,
                 from /home/hans/work/build/media_build/v4l/compat.h:9,
                 from <command-line>:0:
/home/hans/work/build/media_build/v4l/videobuf2-core.c: In function 'vb2_mmap':
include/linux/dynamic_debug.h:60:9: warning: 'plane' may be used uninitialized in this function [-Wmaybe-uninitialized]
   printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__);  \
         ^
/home/hans/work/build/media_build/v4l/videobuf2-core.c:2381:23: note: 'plane' was declared here
  unsigned int buffer, plane;
                       ^
In file included from include/linux/kernel.h:20:0,
                 from include/linux/cache.h:4,
                 from include/linux/time.h:7,
                 from include/linux/input.h:13,
                 from /home/hans/work/build/media_build/v4l/compat.h:9,
                 from <command-line>:0:
include/linux/dynamic_debug.h:60:9: warning: 'buffer' may be used uninitialized in this function [-Wmaybe-uninitialized]
   printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__);  \
         ^
/home/hans/work/build/media_build/v4l/videobuf2-core.c:2381:15: note: 'buffer' was declared here
  unsigned int buffer, plane;
               ^

While these warnings are bogus (the call to __find_plane_by_offset will
set buffer and plane), it doesn't hurt to initialize these variables.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>