drm/xe: Refactor emit_xy_fast_copy and emit_mem_copy functions

drm/xe: Refactor emit_xy_fast_copy and emit_mem_copy functions

To perform copy, based on whether the platform supports service copy
engines, either MEM_COPY or XY_FAST_COPY_BLT instruction is used.
Length of both the instructions is same today and so they use a common
define EMIT_COPY_DW.
This is not true for the future platforms. Implement separate functions
which return the length of the instruction to help in preparing for it.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patch.msgid.link/20260511123746.616662-8-balasubramani.vivekanandan@intel.com
Signed-off-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>