drm/i915: Remove unused enable_cmd_parser modparam

The command parser is feature complete, stable and required by
userspace. In commit 41736a8e ("drm/i915: Use the precomputed value
for whether to enable command parsing") I accidentally removed control
from the modparam, and as no one has complained, remove the left
over modparam completely!

References: 41736a8e

 ("drm/i915: Use the precomputed value for whether to enable command parsing")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180517150727.10431-1-chris@chris-wilson.co.uk