[media] davinci: vpif: Fix module build for capture and display

export the symbols which are used by two modules vpif_capture and
vpif_display. renamed "ch_params" to "vpif_ch_params" so as to avoid
name collision.
This patch fixes following error:
ERROR: "ch_params" [drivers/media/platform/davinci/vpif_display.ko] undefined!
ERROR: "vpif_ch_params_count" [drivers/media/platform/davinci/vpif_display.ko] undefined!
ERROR: "vpif_base" [drivers/media/platform/davinci/vpif_display.ko] undefined!
ERROR: "ch_params" [drivers/media/platform/davinci/vpif_capture.ko] undefined!
ERROR: "vpif_ch_params_count" [drivers/media/platform/davinci/vpif_capture.ko] undefined!
ERROR: "vpif_base" [drivers/media/platform/davinci/vpif_capture.ko] undefined!
make[1]: *** [__modpost] Error 1

Reported-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>