sna: Move initialisation of loop counter to common code

It was only being defined in x86-64 and left unitialized for x86-32.

Fixes regression from
commit 587c486665
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Mon Nov 4 15:10:40 2013 +0000

    sna: Promote uint16_t to a full int to avoid overflow in computing w*h in memcpy_xor

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71286
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/blt.c

@@ -1000,13 +1000,13 @@ memcpy_xor(const void *src, void *dst, int bpp,
 				or |= or << 16;
 			}
 		case 4:
-#if USE_SSE2
 			w = width;
 			if (w * 4 == dst_stride && dst_stride == src_stride) {
 				w *= height;
 				height = 1;
 			}
 
+#if USE_SSE2
 			if (have_sse2()) {
 				do {
 					uint32_t *d = (uint32_t *)dst_bytes;