pvr: Split out unioned structs from struct pvr_sub_cmd

The first patch is a simple optimization to make type-specific uses of struct pvr_sub_cmd slightly less verbose.

The second leverages the new types to require a pointer to to be passed into functions which expect the current sub-command to be of a specific type.

Signed-off-by: Matt Coster <matt.coster@imgtec.com>
Reviewed-by: Rajnesh Kanwal <rajnesh.kanwal@imgtec.com>