drm/imagination: Fixed infinite loop in pvr_vm_mips_map()

Unwinding loop in error path for this function uses unsigned limit
variable, causing the promotion of the signed counter variable.

--> 204         for (; pfn >= start_pfn; pfn--)
                       ^^^^^^^^^^^^^^^^
If start_pfn can be zero then this is an endless loop.  I've seen this
code in other places as well.  This loop is slightly off as well.  It
should decrement pfn on the first iteration.

Fix by making the loop limit variables signed. Also fix missing
predecrement by modifying to while loop.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Donald Robson <donald.robson@imgtec.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231208160825.92933-1-donald.robson@imgtec.com