pvr: Fix image to buffer copies

pvr: Fix image to buffer copies

The code to increment the destination buffer address was wrong. Instead of
incrementing an address on every iteration, the buffer slice size was added
as a static offset to all blits.

Seen in dEQP-VK.draw.renderpass.depth_clamp.d16_unorm, though issues remain.

Signed-off-by: James Glanville <james.glanville@imgtec.com>
Signed-off-by: Karmjit Mahil <Karmjit.Mahil@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22747>