drm: xlnx: zynqmp: tpg should use swap_coeffs as csc matrix

The Test Pattern Generator in video layer creates output in rgb mode,
so in function zynqmp_disp_blend_layer_coeff, we should add a condition
check to ensure that the matrix swap_coeffs for rgb mode is used for tpg
but not sdtv_coeffs.

And delete zynqmp_disp_blend_layer_coeff from zynqmp_disp_layer_set_tpg,
this is because that zynqmp_disp_blend_layer_coeff will be called later
by zynqmp_disp_layer_enable.

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>