staging: sm750fb: Use kzalloc rather than kmalloc followed by memset with 0

The semantic patch that makes this change is available
in scriptcoccinelle/api/alloc/kzalloc-simple.cocci.

Signed-off-by: Madhusudhanan Ravindran <mravindran04@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>