drm/i915/dg1: Increase mmio size to 4MB

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