drm/vc4: Fix sleeps during the IRQ handler for DSI transactions.

VC4's DSI1 has a bug where the AXI connection is broken for 32-bit
writes from the CPU, so we use the DMA engine to DMA 32-bit values
into registers instead.  That sleeps, so we can't do it from the top
half.

As a solution, use an interrupt thread so that all our writes happen
when sleeping is is allowed.

v2: Use IRQF_ONESHOT (suggested by Boris)
v3: Style nitpicks.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20171014001255.32005-1-eric@anholt.net
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com> (v2)