drm/i915/dg1: Update DMC_DEBUG register

Update the DMC_DEBUG_DC5 register to its new location and do not try
reading the DC6 counter since DG1 doesn't support DC6.

v2: Use IS_DGFX() instead of IS_DG1(). Even if not having DC6 is not
directly related to DGFX, the register move to a new location is. So in
future, if there is one supporting DC6, it would just need to add the
other register rather than fixing the case of a wrong register being
read (Matt)

Cc: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201014191937.1266226-10-lucas.demarchi@intel.com