drm/i915/gvt: use macros from drm_dp.h instead of duplication

drm/i915/gvt: use macros from drm_dp.h instead of duplication

Use the existing macros in drm_dp.h for DPCD and DP AUX instead of
duplicating. Remove unused macros, as well as the duplicate definition
of DPCD_SIZE.

AUX_NATIVE_REPLY_NAK is left unchanged, as it does not match
DP_AUX_NATIVE_REPLY_NACK, and I'm not sure what the right thing to do is
here.

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Zhi Wang <zhiwang@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240930135342.3562755-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>