staging: sm750fb: share common bits in display control registers

The display control registers for primary and secondary display share some
of the bits and those bits can be defined in a single place and then used
for manipulations of the relevant registers.

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