drm/imagination: Ensure PVR_MIPS_PT_PAGE_COUNT is never zero

When the host page size was more than 4 times larger than the FW page
size, this macro evaluated to zero resulting in zero-sized arrays.

Use DIV_ROUND_UP() to ensure the correct behavior.

Reported-by:  <20240228012313.5934-1-yaolu@kylinos.cn>
Closes: https://lore.kernel.org/dri-devel/20240228012313.5934-1-yaolu@kylinos.cn
Link: https://lore.kernel.org/dri-devel/20240228012313.5934-1-yaolu@kylinos.cn


Fixes: 927f3e02 ("drm/imagination: Implement MIPS firmware processor and MMU support")
Cc: stable@vger.kernel.org
Signed-off-by: Matt Coster <matt.coster@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>