drm/amd/display: Use macro for isnan check

In code provided by HW teams we do a NaN check on floats
by comparing the number against itself. This confuses most
people including myself. Macro it out to make it self-explanatory.

Don't do a NaN check for int.

v2: parantheses around 'number' expression

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>