drm/i915: Rename FORCEWAKE_BLITTER to FORCEWAKE_GT

drm/i915: Rename FORCEWAKE_BLITTER to FORCEWAKE_GT

The power well that we've been referring to as the 'blitter' well is
actually more of a general GT power well which contains a lot of things
other than the blitter engine registers.  The FORCEWAKE_BLITTER name in
the code was used for historic reasons, but no longer matches how the
bspec describes this power well and just causes confusion for people not
familiar with this area of the code.  Let's rename it to FORCEWAKE_GT to
more accurately describe the role of the power well and match how the
modern bspec refers to it.

v2:
 - Add a comment noting that the GT power well includes the blitter
   engine. (Jose)

Bspec: 66696, 66534, 67609
Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201009194442.3668677-2-matthew.d.roper@intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>