drm: tests: Fix a buffer overflow in format_helper_test

The xrgb2101010 format conversion test (unlike for other formats) does
an endianness conversion on the results. However, it always converts
TEST_BUF_SIZE 32-bit integers, which results in reading from (and
writing to) more memory than in present in the result buffer. Instead,
use the buffer size, divided by sizeof(u32).

The issue could be reproduced with KASAN:
./tools/testing/kunit/kunit.py run --kunitconfig drivers/gpu/drm/tests \
	--kconfig_add CONFIG_KASAN=y --kconfig_add CONFIG_KASAN_VMALLOC=y \
	--kconfig_add CONFIG_KASAN_KUNIT_TEST=y \
	drm_format_helper_test.*xrgb2101010

Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Fixes: 45311431 ("drm/format-helper: Add KUnit tests for drm_fb_xrgb8888_to_xrgb2101010()")
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221019073239.3779180-1-davidgow@google.com