mesa/st: Fix PIPE_FORMAT_R8G8B8A8_SRGB -> MESA_FORMAT_ conversion.

Copy'n'past typo introduced in my
1d8e3067fd commit.

This fixes swapped RB channels I was seeing in my test machines.

Trivial.

src/mesa/state_tracker/st_format.c

@@ -754,7 +754,7 @@ st_pipe_format_to_mesa_format(enum pipe_format format)
    case PIPE_FORMAT_B8G8R8X8_SRGB:
       return MESA_FORMAT_B8G8R8X8_SRGB;
    case PIPE_FORMAT_R8G8B8A8_SRGB:
-      return MESA_FORMAT_B8G8R8A8_SRGB;
+      return MESA_FORMAT_R8G8B8A8_SRGB;
 
    default:
       assert(0);