modetest: fix some compiler warnings

Use unsigned int instead of int:
- modetest.c:90:1: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
- modetest.c:98:1: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
- modetest.c:118:1: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
- modetest.c:286:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
- modetest.c:303:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
- modetest.c:694:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
- modetest.c:1088:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

The 'fd' variable is global, we don't need to pass it as an argument:
- modetest.c:998:40: warning: unused parameter ‘fd’ [-Wunused-parameter]

We don't use the 'modeset' variable:
- modetest.c:1025:8: warning: variable ‘modeset’ set but not used [-Wunused-but-set-variable]

V2: rebase, clear some more warnings

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>