drm/msm/dsi/phy: Do not overwite PHY_CMN_CLK_CFG1 when choosing bitclk source

drm/msm/dsi/phy: Do not overwite PHY_CMN_CLK_CFG1 when choosing bitclk source

PHY_CMN_CLK_CFG1 register has four fields being used in the driver: DSI
clock divider, source of bitclk and two for enabling the DSI PHY PLL
clocks.

dsi_7nm_set_usecase() sets only the source of bitclk, so should leave
all other bits untouched.  Use newly introduced
dsi_pll_cmn_clk_cfg1_update() to update respective bits without
overwriting the rest.

While shuffling the code, define and use PHY_CMN_CLK_CFG1 bitfields to
make the code more readable and obvious.

Fixes: 1ef7c99d145c ("drm/msm/dsi: add support for 7nm DSI PHY/PLL")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/637380/
Link: https://lore.kernel.org/r/20250214-drm-msm-phy-pll-cfg-reg-v3-3-0943b850722c@linaro.org
Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>