drm/ttm: Add a parameter to add extra pages into ttm_tt

Add a parameter called "extra_pages" for ttm_tt_init, to indicate that
driver needs extra pages in ttm_tt.

v2:
  Used imperative wording [Thomas and Christian]

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
cc: Christian Koenig <christian.koenig@amd.com>
cc: Hellstrom Thomas <thomas.hellstrom@intel.com>
Reviewed-by: Thomas Hellstrom <thomas.hellstrom@linux.intel.com>
Reviewed-by: Christian Konig <christian.koenig@amd.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220401123751.27771-8-ramalingam.c@intel.com