gfx: gtt: refcount gtt mappings

gfx: gtt: refcount gtt mappings

Currently psb_gtt_map_* can handle multiple calls with the same
handle but will remove the pages from gtt as soon as one call to
psb_gtt_unmap_* is made. Since multiple drm_fb's can be added with
the same handle and the driver will cal psb_gtt_map_meminfo for each
one of them, this could lead to unmapping memory that is still being
used.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>