drm/vkms: fix xrgb on compute crc

The previous memset operation was not correctly zeroing the alpha
channel to compute the crc, and as a result, the IGT subtest
kms_cursor_crc/pipe-A-cursor-alpha-transparent fails.

Fixes: db7f419c ("drm/vkms: Compute CRC with Cursor Plane")
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200730202524.5upzuh4irboru7my@smtp.gmail.com

drivers/gpu/drm/vkms/vkms_composer.c

@@ -33,7 +33,7 @@ static uint32_t compute_crc(void *vaddr_out, struct vkms_composer *composer)
 				     + (i * composer->pitch)
 				     + (j * composer->cpp);
 			/* XRGB format ignores Alpha channel */
-			memset(vaddr_out + src_offset + 24, 0,  8);
+			bitmap_clear(vaddr_out + src_offset, 24, 8);
 			crc = crc32_le(crc, vaddr_out + src_offset,
 				       sizeof(u32));
 		}