osmesa: Fix osmesa_context.DataType type.

osmesa: Fix osmesa_context.DataType type.

Fixes these GCC warnings.
osmesa.c: In function ‘osmesa_renderbuffer_storage’:
osmesa.c:417: warning: comparison is always false due to limited range of data type
osmesa.c:423: warning: comparison is always false due to limited range of data type
osmesa.c:431: warning: comparison is always false due to limited range of data type
osmesa.c:437: warning: comparison is always false due to limited range of data type
osmesa.c:447: warning: comparison is always false due to limited range of data type
osmesa.c:453: warning: comparison is always false due to limited range of data type
osmesa.c:463: warning: comparison is always false due to limited range of data type
osmesa.c:466: warning: comparison is always false due to limited range of data type
osmesa.c:476: warning: comparison is always false due to limited range of data type
osmesa.c:479: warning: comparison is always false due to limited range of data type

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Signed-off-by: Brian Paul <brianp@vmware.com>