OMAPDSS: DSS: Remove redundant functions

Functions dss_calc_clock_rates() and dss_get_clock_div() are removed as these
functions have become redundant and no longer used.

Signed-off-by: Chandrabhanu Mahapatra <cmahapatra@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>