drm/i915/guc: Prepare to process incoming requests from CT

Requests are read from CT in the irq handler, but actual processing
will be done in the work thread. Processing of specific actions will
be added in the upcoming patches.

v2: don't use GEM_BUG_ON (Chris)
    don't kmalloc too large buffer (Michal)
v3: rebased
v4: don't name it 'dispatch' (Michel) and fix checkpatch
    add some documentation (Michal)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Oscar Mateo <oscar.mateo@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180326194829.58836-10-michal.wajdeczko@intel.com