vf_ssim: Fix loading doubles to float registers on i386

This fixes the tests filter-refcmp-ssim-yuv and filter-refcmp-ssim-rgb
on i386 after breaking in fcc0424c.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavfilter/x86/vf_ssim.asm

@@ -255,6 +255,6 @@ cglobal ssim_end_line, 3, 3, 7, sum0, sum1, w
     addpd             m0, m4
 %if ARCH_X86_32
     movsd            r0m, m0
-    fld             r0mp
+    fld        qword r0m
 %endif
     RET