drm/imagination: avoid -Woverflow warning

The array size calculation in pvr_vm_mips_fini() appears to be incorrect based on
taking the size of the pointer rather than the size of the array, which manifests
as a warning about signed integer overflow:

In file included from include/linux/kernel.h:16,
                 from drivers/gpu/drm/imagination/pvr_rogue_fwif.h:10,
                 from drivers/gpu/drm/imagination/pvr_ccb.h:7,
                 from drivers/gpu/drm/imagination/pvr_device.h:7,
                 from drivers/gpu/drm/imagination/pvr_vm_mips.c:4:
drivers/gpu/drm/imagination/pvr_vm_mips.c: In function 'pvr_vm_mips_fini':
include/linux/array_size.h:11:25: error: overflow in conversion from 'long unsigned int' to 'int' changes value from '18446744073709551615' to '-1' [-Werror=overflow]
   11 | #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))
      |                         ^
drivers/gpu/drm/imagination/pvr_vm_mips.c:106:24: note: in expansion of macro 'ARRAY_SIZE'
  106 |         for (page_nr = ARRAY_SIZE(mips_data->pt_pages) - 1; page_nr >= 0; page_nr--) {
      |                        ^~~~~~~~~~

Just use the number of array elements directly here, and in the corresponding
init function for consistency.

Fixes: 927f3e02 ("drm/imagination: Implement MIPS firmware processor and MMU support")
Reviewed-by: Donald Robson <donald.robson@imgtec.com>
Link: https://lore.kernel.org/lkml/9df9e4f87727399928c068dbbf614c9895ae15f9.camel@imgtec.com/


Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Matt Coster <matt.coster@imgtec.com>