gallium/aux/tgsi_exec.c: Fix various -Wsign-compare

gallium/aux/tgsi_exec.c: Fix various -Wsign-compare

tgsi/tgsi_exec.c: In function 'exec_tex':
tgsi/tgsi_exec.c:2254:46: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
       assert(shadow_ref >= dim && shadow_ref < ARRAY_SIZE(args));
                                              ^
./util/u_debug.h:189:30: note: in definition of macro 'debug_assert'
 #define debug_assert(expr) ((expr) ? (void)0 : _debug_assert_fail(#expr,
__FILE__, __LINE__, __FUNCTION__))
                              ^~~~
tgsi/tgsi_exec.c:2254:7: note: in expansion of macro 'assert'
       assert(shadow_ref >= dim && shadow_ref < ARRAY_SIZE(args));
       ^~~~~~
tgsi/tgsi_exec.c:2290:23: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
       for (i = dim; i < ARRAY_SIZE(args); i++)
                       ^
In file included from ./util/u_memory.h:39:0,
                 from tgsi/tgsi_exec.c:62:
tgsi/tgsi_exec.c: In function 'exec_lodq':
tgsi/tgsi_exec.c:2357:15: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
    assert(dim <= ARRAY_SIZE(coords));
               ^
./util/u_debug.h:189:30: note: in definition of macro 'debug_assert'
 #define debug_assert(expr) ((expr) ? (void)0 : _debug_assert_fail(#expr,
__FILE__, __LINE__, __FUNCTION__))
                              ^~~~
tgsi/tgsi_exec.c:2357:4: note: in expansion of macro 'assert'
    assert(dim <= ARRAY_SIZE(coords));
    ^~~~~~
tgsi/tgsi_exec.c:2363:20: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
    for (i = dim; i < ARRAY_SIZE(coords); i++) {
                    ^

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>