tools/power/x86/intel-speed-select: Introduce isst_get_disp_freq_multiplier

Remove hardcoded DISP_FREQ_MULTIPLIER in the code and use
isst_get_disp_freq_multiplier() instead.

No functional changes are expected.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>