drm/dp/mst: Replace the fixed point thing with straight calculation

Get rid of the drm_fixp_from_fraction() usage and just do the
straightforward calculation directly.

Cc: Lyude Paul <lyude@redhat.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190925141442.23236-3-ville.syrjala@linux.intel.com
Reviewed-by: Lyude Paul <lyude@redhat.com>