drm/xe: Refactor emit_clear_main_copy

drm/xe: Refactor emit_clear_main_copy

Implement a function which returns the length of XY_FAST_COLOR_BLT
instruction instead of hardcoding it inside the emit_clear_main_copy.
In future platforms, the length of this instruction is expected to
change and this patch helps in preparing for it.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patch.msgid.link/20260511123746.616662-6-balasubramani.vivekanandan@intel.com
Signed-off-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>