drm/panthor: Use the BITS_PER_LONG macro

sizeof(unsigned long) * 8 is the number of bits in an unsigned long
variable, replace it with BITS_PER_LONG macro to make them simpler.

And fix the warning:
	WARNING: Comparisons should place the constant on the right side of the test
	#23: FILE: drivers/gpu/drm/panthor/panthor_mmu.c:2696:
	+       if (BITS_PER_LONG < va_bits) {

Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240902094404.1943710-1-ruanjinjie@huawei.com