msm_fb: display; Fix snprintf length usage

Using a newer GCC we get the following warning:

drivers/video/msm/mipi_novatek.c: In function 'mipi_dsi_3d_barrier_read':
drivers/video/msm/mipi_novatek.c:683:37: warning: argument to 'sizeof' in
'snprintf' call is the same expression as the destination; did you mean to
provide an explicit length? [-Wsizeof-pointer-memaccess]

These look to be due to using sizeof(buf) which is a pointer,
access it properly.

Signed-off-by: Zhao Wei Liew <zhaoweiliew@gmail.com>