va_x11: fix klockscan issue (details is bellow)

va_x11: fix klockscan issue (details is bellow)

File:        va_x11.c
Function:    va_DisplayContextGetDriverName -> va_DRI2GetDriverName -> dri2_util.c:isDRI2Connected
Line:        154 -> 84 -> dri2_util.c:184
Code:
TRACEBACK
va_x11.c:151: driver_name is NULL
va_x11.c:154: 'driver_name' is passed to function 'va_DRI2GetDriverName'.
dri2_util.c:84: 'driver_name' is passed to function 'isDRI2Connected'.
dri2_util.c:184: 'driver_name' is explicitly dereferenced.

Signed-off-by: Austin Yuan <shengquan.yuan@gmail.com>