drm/xe/topology: Add a function to find the index of the last enabled DSS in a mask

Last enabled DSS in a DSS mask can help estimate the maximum DSSes enabled
in the DSS mask, as the enabled DSSes can be discontiguous.

Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Harish Chegondi <harish.chegondi@intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/79944bb27eb4f7ce5df01f964aebbf431b3a6c61.1740533885.git.harish.chegondi@intel.com