media: rockchip/rga: use 64-bit arithmetic instead of 32-bit

Cast p to dma_addr_t in order to avoid a potential integer overflow.
This variable is being used in a context that expects an expression
of type dma_addr_t (u64).

The expression p << PAGE_SHIFT is currently being evaluated
using 32-bit arithmetic.

Addresses-Coverity-ID: 1458347 ("Unintentional integer overflow")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>