hevcdsp_idct_neon.S: Avoid unnecessary mov.

ret can be given an argument instead.
This is also consistent with how other assembler code
in FFmpeg does it.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavcodec/aarch64/hevcdsp_idct_neon.S

@@ -617,8 +617,7 @@ function ff_hevc_idct_16x16_\bitdepth\()_neon, export=1
 
         add              sp,  sp,  #640
 
-        mov             x30, x15
-        ret
+        ret             x15
 endfunc
 .endm
 
@@ -814,8 +813,7 @@ function ff_hevc_idct_32x32_\bitdepth\()_neon, export=1
 .endr
 
         add             sp,  sp,  #2432
-        mov             x30, x15
-        ret
+        ret             x15
 endfunc
 .endm