Extract the gradient related code out.

Extract the gradient related code out.

 1. Extract the logic of gradient from the glamor_render.c
 to the file glamor_gradient.c.
 2. Modify the logic of gradient pixmap gl draw. Use the
 logic like composite before, but the gradient always just
 have one rect to render, so no need to set the VB and EB,
 replace it with just call glDrawArrays. 3.Kill all the
 warning in glamor_render.c

Reviewed-by: Zhigang Gong<zhigang.gong@linux.intel.com>
Signed-off-by: Junyan He <junyan.he@linux.intel.com>
Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>