staging: sm750fb: remove hw_sm750_getVMSize function

The hw_sm750_getVMSize wraps ddk750_getVMSize without any added
functionality. Call ddk750_getVMSize directly and remove
hw_sm750_getVMSize.

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