arm: vp9mc: Use a different helper register for PIC loads

This fixes crashes since 557c1675 in linux PIC builds.

Previously, movrelx silently used r12 as helper register, which
doesn't work when r12 is the destination register.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/arm/vp9mc_neon.S

@@ -404,7 +404,7 @@ function ff_vp9_\type\()_\filter\()\size\()_h_neon, export=1
         ldr             r4,  [sp, #16]
         ldr             r5,  [sp, #20]
 .endif
-        movrelx         r12, X(ff_vp9_subpel_filters)
+        movrelx         r12, X(ff_vp9_subpel_filters), r6
         add             r12, r12, 120*\offset - 8
         cmp             r5,  #8
         add             r12, r12, r5, lsl #3
@@ -680,8 +680,8 @@ function ff_vp9_\type\()_\filter\()\size\()_v_neon, export=1
         push            {r4-r5}
         vpush           {q4-q7}
         ldr             r4,  [sp, #72]
+        movrelx         r12, X(ff_vp9_subpel_filters), r5
         ldr             r5,  [sp, #80]
-        movrelx         r12, X(ff_vp9_subpel_filters)
         add             r12, r12, 120*\offset - 8
         add             r12, r12, r5, lsl #3
         cmp             r5,  #8