media: platform: omap: Pass a pointer to virt_to_page()

A pointer into virtual memory is represented by a (void *)
not an u32, so the compiler warns:

drivers/media/platform/ti/omap/omap_voutlib.c:317:54: warning:
  passing argument 1 of 'virt_to_pfn' makes pointer from integer
  without a cast [-Wint-conversion]

Fix this with an explicit cast.

Cc: Vaibhav Hiremath <hvaibhav.linux@gmail.com>
Cc: Archit Taneja <architt@codeaurora.org>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>