staging: sm750fb: refactor setDisplayControl function

The enable/disbable sequence in setDisplayControl function is duplicated
for primary and secondary display controllers. The function can be
refactored so that the common part of register access will be shared for
both controllers.

Signed-off-by: Mike Rapoport <mike.rapoport@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>