intel/bufmgr: Cast bitshift to unsigned

intel/bufmgr: Cast bitshift to unsigned

../src/mesa/drivers/dri/i965/intel_buffer_objects.c:405:4: runtime error: left shift of 255 by 24 places cannot be represented in type 'int'
    #0 0x7f9404ac4ae1 in brw_map_buffer_range ../src/mesa/drivers/dri/i965/intel_buffer_objects.c:405
    #1 0x7f9405a9cb13 in vbo_save_map_vertex_store ../src/mesa/vbo/vbo_save_api.c:261
    #2 0x7f9405b6a89d in vbo_save_NewList ../src/mesa/vbo/vbo_save_api.c:1774
    #3 0x7f94051aba3d in _mesa_NewList ../src/mesa/main/dlist.c:14172

../src/gallium/drivers/iris/iris_resource.c:1725:61: runtime error: left shift of 255 by 24 places cannot be represented in type 'int'
    #0 0x7fe51c820c8e in iris_map_direct ../src/gallium/drivers/iris/iris_resource.c:1725
    #1 0x7fe51c82322c in iris_transfer_map ../src/gallium/drivers/iris/iris_resource.c:1895
    #2 0x7fe5202628be in u_transfer_helper_transfer_map ../src/gallium/auxiliary/util/u_transfer_helper.c:243
    #3 0x7fe51997c508 in pipe_buffer_map_range ../src/gallium/auxiliary/util/u_inlines.h:344
    #4 0x7fe51997ec8d in u_upload_alloc_buffer ../src/gallium/auxiliary/util/u_upload_mgr.c:221
    #5 0x7fe51997f24f in u_upload_alloc ../src/gallium/auxiliary/util/u_upload_mgr.c:254
    #6 0x7fe51ccf43af in upload_state ../src/gallium/drivers/iris/iris_state.c:323
    #7 0x7fe51d06963a in gen9_init_state ../src/gallium/drivers/iris/iris_state.c:7516
    #8 0x7fe51c7c2ea0 in iris_create_context ../src/gallium/drivers/iris/iris_context.c:294
    #9 0x7fe519dc729b in st_api_create_context ../src/mesa/state_tracker/st_manager.c:921
    #10 0x7fe5198c47ea in dri_create_context ../src/gallium/state_trackers/dri/dri_context.c:161
    #11 0x7fe519898aac in driCreateContextAttribs ../src/mesa/drivers/dri/common/dri_util.c:475

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3825>