drm/amd/display: Move bw_fixed macros to header file

The macros defined at bw_fixed are important mathematical definitions,
specifying masks to get the fractional part and the maximum and minimum
values of I64. In order to enable unit tests for bw_fixed, it is
relevant to have access to those macros. This commit moves the macros to
the header file, making it accessible to future unit tests.

Signed-off-by: Maíra Canal <maira.canal@usp.br>