msm: mdss: Force PP block to match FB device during ioctl

In the postprocessing ioctl, there is a configurable block parameter
which can be used to choose the logical display for the config. However,
the framebuffer device opened for the ioctl should be the display for
the config. Rather than deprecating the block parameter, this change
forces the block parameter to match the FB device used for the ioctl.

Change-Id: I76e100311ff8f3361e9c7b7759de78becd332685
Signed-off-by: Benet Clark <benetc@codeaurora.org>