drm/i915: Create a kernel context for GGTT updates

Create a separate kernel context if a platform requires
GGTT updates using MI_UPDATE_GTT blitter command.

Subsequent patch will introduce methods to update
GGTT using this bind context and MI_UPDATE_GTT blitter
command.

v2: fix context leak on err(Oak)
v3: improve err handling and improve function names(Andi)
    add docs for few functions.

Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Oak Zeng <oak.zeng@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230926083742.14740-3-nirmoy.das@intel.com