drm/xe: Allow userspace to configure CCS mode

Allow user to configure the CCS mode setting through a
'ccs_mode' sysfs interface. Also report the current
CCS mode configuration and number of compute slices
available through this interface.

v2: Rebase, make it platform agnostic
v3: Separte out num_cslices sysfs interface and make
    xe_gt_ccs_mode_sysfs_init() return void

Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>