video: Use bool instead int pointer for get_opt_bool() argument

As the function name already indicates that get_opt_bool() parses
for a bool. It is not a surprise that compiler is complaining
about it when -Werror=incompatible-pointer-types is used:

drivers/video/fbdev/intelfb/intelfbdrv.c: In function ‘intelfb_setup’:
drivers/video/fbdev/intelfb/intelfbdrv.c:353:39: error: passing argument 3 of ‘get_opt_bool’ from incompatible pointer type [-Werror=incompatible-pointer-types]
   if (get_opt_bool(this_opt, "accel", &accel))

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>