drm/i915: add lmem_size modparam

lmem_size is used to limit the amount of lmem for testing purposes.
Default is to use hardware available lmem size.

Signed-off-by: CQ Tang <cq.tang@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Nirmoy Das <nirmoy.das@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220324143123.348590-2-matthew.auld@intel.com