drm/format-helper: Use drm_format_info_min_pitch() in tests helper

There's a nice macro to calculate the destination pitch that already takes
into account sub-byte pixel formats. Use that instead of open coding it.
Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230316223404.102806-1-javierm@redhat.com

drivers/gpu/drm/tests/drm_format_helper_test.c

@@ -440,15 +440,12 @@ static size_t conversion_buf_size(u32 dst_format, unsigned int dst_pitch,
 				  const struct drm_rect *clip)
 {
 	const struct drm_format_info *dst_fi = drm_format_info(dst_format);
-	unsigned int bpp;
 
 	if (!dst_fi)
 		return -EINVAL;
 
-	if (!dst_pitch) {
-		bpp = drm_format_info_bpp(dst_fi, 0);
-		dst_pitch = DIV_ROUND_UP(drm_rect_width(clip) * bpp, 8);
-	}
+	if (!dst_pitch)
+		dst_pitch = drm_format_info_min_pitch(dst_fi, 0, drm_rect_width(clip));
 
 	return dst_pitch * drm_rect_height(clip);
 }