gallium: include u_format.h for util_format_is_supported

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...