pvr: Fix cubemap layer stride

pvr: Fix cubemap layer stride

The hw calculates the layer stride with the assumption of a full
mip chain being there even though certain levels might never be
used.

Fix the `layer_size` by accounting for any missing mip levels.

Fixes: 8991e6464 ("pvr: Add a Vulkan driver for Imagination Technologies PowerVR Rogue GPUs")
Reported-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/25344>