video: msmfb: Put the partial update magic value into the fix_screen struct.

This can then be tested by userspace to see if the capability is supported.
Userspace cannot rely on that value being left in var_screen, since userspace
itself can change it.

Signed-off-by: Dima Zavin <dima@android.com>
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>