drm/msm/dsi: save PLL registers across first PHY reset

Take advantage of previously-added support for persisting PLL
registers across DSI PHY disable/enable cycles (see 328e1a63
'drm/msm/dsi: Save/Restore PLL status across PHY reset') to
support persisting across the very first DSI PHY enable at
boot.

The bootloader may have left the PLL registers in a non-default
state. For example, for dsi_pll_28nm.c on 8x16/8x39, the byte
clock mux's power-on reset configuration is to bypass DIV1, but
depending on bandwidth requirements[1] the bootloader may have
set the DIV1 path.

When the byte clock mux is registered with the generic clock
framework at probe time, the framework reads & caches the value
of the mux bit field (the initial clock parent). After PHY enable,
when clk_set_rate is called on the byte clock, the framework
assumes there is no need to reparent, and doesn't re-write the
mux bit field. But PHY enable resets PLL registers, so the mux
bit field actually silently reverted to the DIV1 bypass path.
This causes the byte clock to be off by a factor of e.g. 2 for
our tested WXGA panel.

The above issue manifests as the display not working and a
constant stream of FIFO/LP0 contention errors.

[1] The specific requirement for triggering the DIV1 path (and
thus this issue) on 28nm is a panel with pixel clock <116.7MHz
(one-third the minimum VCO setting). FHD/1080p (~145MHz) is fine,
WXGA/1280x800 (~75MHz) is not.

Signed-off-by: Benjamin Li <benl@squareup.com>