staging: sm750fb: Remove spinlock helper function

This patch removes the unnecessary spinlock helper function and instead
calls spin_lock and spin_unlock directly.

This does *not* resolve sparse warnings about context imbalances but these are
spurious.

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>