drm/mgag200: Split PLL setup into compute and update functions

The _set_plls() functions compute a pixel clock's PLL values
and program the hardware accordingly. This happens during atomic
commits.

For atomic modesetting, it's better to separate computation and
programming from each other. This will allow to compute the PLL
value during atomic checks and catch unsupported modes early.

Split the PLL setup into a compute and an update functions, and
call them one after the other. Computed PLL values are store in
struct mgag200_pll_values. There are four parameters for the PLL,
m, n, p and s. Every compute function stores a value for each
of these parameters, and the rsp update function makes the register
bits from them. The values stored by the compute function are
either plain values or register bits. An additional change is
required to always store plain values.

No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714142240.21979-5-tzimmermann@suse.de

drivers/gpu/drm/mgag200/mgag200_drv.h

@@ -126,6 +126,23 @@
 #define MGAG200_MAX_FB_HEIGHT 4096
 #define MGAG200_MAX_FB_WIDTH 4096
 
+/*
+ * Stores parameters for programming the PLLs
+ *
+ * Fref: reference frequency (A: 25.175 Mhz, B: 28.361, C: XX Mhz)
+ * Fo: output frequency
+ * Fvco = Fref * (N / M)
+ * Fo = Fvco / P
+ *
+ * S = [0..3]
+ */
+struct mgag200_pll_values {
+	unsigned int m;
+	unsigned int n;
+	unsigned int p;
+	unsigned int s;
+};
+
 #define to_mga_connector(x) container_of(x, struct mga_connector, base)
 
 struct mga_i2c_chan {