drm/amd/display: fix dml log2 function

This change removes internal rounding in dml_log2 function.

Dml_log2 is expected to return a float output. In case an int is needed
dml will floor the output on it's own.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Eric Bernstein <Eric.Bernstein@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>