drm/amd/display: simplify dml log2 function

Current implementation is slightly inaccurate and will often
result in truncation/floor operation decrementing an exact
integer output by 1.

Only rounded down output is ever expected, just extract the fp
exponent for this to increase performance and avoid any
truncation issues.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Eric Bernstein <Eric.Bernstein@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>