radeonsi: use shifts for sign extension

Avoids a branch and reduces code size a tiny bit:
    text   data     bss      dec    hex filename
10804563 398653 2070368 13273584 ca89f0 /tmp/radeonsi_dri.so.old
10804499 398653 2070368 13273520 ca89b0 /tmp/radeonsi_dri.so

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/drivers/radeonsi/si_descriptors.c

@@ -100,8 +100,8 @@ static uint64_t si_desc_extract_buffer_address(const uint32_t *desc)
 		      ((uint64_t)G_008F04_BASE_ADDRESS_HI(desc[1]) << 32);
 
 	/* Sign-extend the 48-bit address. */
-	if (va & (1ull << 47))
-		va |= 0xffffull << 48;
+	va <<= 16;
+	va = (int64_t)va >> 16;
 	return va;
 }