On DGFX the register range has been extended to go up to 8MB. However we
only actually use up to address 280000h, so let's increase it to 4MB.

v2 (Lucas):  add bspec reference and reword commit message to explain
   the 4 vs 8 MB used (requested by Matt Roper)

Bspec: 53616

Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Venkata Sandeep Dhanalakota <venkata.s.dhanalakota@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/20201007002210.3678024-4-lucas.demarchi@intel.com