drm/i915: Generalise common GPU engine reset request/unrequest code

GPU engine reset handshaking is something that is applicable to both full GPU
reset and engine reset, which is something that is part of the upcoming TDR
per-engine hang recovery patches. Break out the common engine reset
request/unrequest code (originally written by Mika Kuoppala) for reuse later
in the TDR enablement patch series.

v2: correct indentation and drop unused returned value (Mika)
v3: We have forcewake during reset so use *_FW reg access (Mika)

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Tomas Elf <tomas.elf@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
[Mika: Fixed format warning]
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456929984-16323-1-git-send-email-mika.kuoppala@intel.com