gallium: include u_format.h for util_format_is_supported

Without it gcc complains:
nv50_screen.c: In function ‘nv50_screen_is_format_supported’:
nv50_screen.c:48: warning: implicit declaration of function ‘util_format_is_supported’

and handles it wrongly - util_format_is_supported returns boolean, which is typedef'ed
to uchar, but function without prototype is assumed to return int.

For me nv50_screen_is_format_supported was returning true for float formats without
--enable-texture-float...

src/gallium/drivers/i915/i915_screen.c

@@ -27,6 +27,7 @@
 
 
 #include "draw/draw_context.h"
+#include "util/u_format.h"
 #include "util/u_inlines.h"
 #include "util/u_memory.h"
 #include "util/u_string.h"

src/gallium/drivers/i965/brw_screen.c

@@ -26,6 +26,7 @@
  **************************************************************************/
 
 
+#include "util/u_format.h"
 #include "util/u_inlines.h"
 #include "util/u_memory.h"
 #include "util/u_string.h"

src/gallium/drivers/nv50/nv50_screen.c

@@ -20,6 +20,7 @@
  * SOFTWARE.
  */
 
+#include "util/u_format.h"
 #include "util/u_format_s3tc.h"
 #include "pipe/p_screen.h"
 

src/gallium/drivers/nvc0/nvc0_screen.c

@@ -20,6 +20,7 @@
  * SOFTWARE.
  */
 
+#include "util/u_format.h"
 #include "util/u_format_s3tc.h"
 #include "pipe/p_screen.h"
 

src/gallium/drivers/nvfx/nvfx_screen.c

@@ -1,5 +1,6 @@
 #include "pipe/p_screen.h"
 #include "pipe/p_state.h"
+#include "util/u_format.h"
 #include "util/u_format_s3tc.h"
 #include "util/u_simple_screen.h"
 

src/gallium/drivers/r600/r600_pipe.c

@@ -30,6 +30,7 @@
 #include <tgsi/tgsi_util.h>
 #include <util/u_blitter.h>
 #include <util/u_double_list.h>
+#include "util/u_format.h"
 #include <util/u_format_s3tc.h>
 #include <util/u_transfer.h>
 #include <util/u_surface.h>