drm/i915/gvt: Fix bug in getting msg length in AUX CH registers handler

Msg length should be obtained from value written to AUX_CH_CTL register
rather than from enum type of the register.

Commit 0cad796a  ("drm/i915: Use REG_BIT() & co. for AUX CH registers")
incorrectly calculates the msg_length from reg type and yields below
warning in intel_gvt_i2c_handle_aux_ch_write():
"i915 0000:00:02.0: drm_WARN_ON(msg_length != 4)".

Fixes: 0cad796a

 ("drm/i915: Use REG_BIT() & co. for AUX CH registers")
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20230731112033.7275-1-yan.y.zhao@intel.com
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>