drm/i915/tgl: Move GTCR register to cope with GAM MMIO address remap

GAM registers located in the 0x4xxx range have been relocated to 0xCxxx;
this is to make space for global MOCS registers.

v2: Rename register and bitfield to its new name (suggested by Mika)

HSD: 399379
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190823082055.5992-2-lucas.demarchi@intel.com