drm/msm/dsi: fix definition of msm_dsi_pll_28nm_8960_init()

This fixes the following build failure:

drivers/gpu/drm/msm/dsi/pll/dsi_pll_28nm.o: In function `msm_dsi_pll_28nm_8960_init':
dsi_pll_28nm.c:(.text+0x1198): multiple definition of `msm_dsi_pll_28nm_8960_init'
drivers/gpu/drm/msm/dsi/pll/dsi_pll.o:dsi_pll.c:(.text+0x0): first defined here

Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
Acked-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>