drm/imagination: Add a per-file PVR context list

This adds a linked list of VM contexts which is needed for the next patch
to be able to correctly track VM contexts for destruction on file close.

It is only safe for VM contexts to be removed from the list and destroyed
when not in interrupt context.

Signed-off-by: Brendan King <brendan.king@imgtec.com>
Signed-off-by: Matt Coster <matt.coster@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Cc: stable@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/e57128ea-f0ce-4e93-a9d4-3f033a8b06fa@imgtec.com