drm/i915: correct name of GT forcewake domain in error messages

For historical reasons, the GT forcewake domain used to be referred to
as the "blitter" domain; that name is no longer accurate since the GT
domain contains a lot of additional registers and functionality besides
just the blitter.  Although we renamed the domain in the driver in
commit 55e3c170 ("drm/i915: Rename FORCEWAKE_BLITTER to
FORCEWAKE_GT"), we neglected to update the string that gets printed in
driver error messages; let's do that now to avoid confusion.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Caz Yokoyama <caz.yokoyama@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210729054118.2458523-2-matthew.d.roper@intel.com