dummy driver obvious DacSpeed fix

[Antoine Martin]

Hello,

The dummy driver doesn't honour the "DacSpeed" config file option.
It sets a local "maxClock" variable, but doesn't use it.
This fixes it, and allowed me to run tests with resolutions up to
20000x20000:

--- a/src/dummy_driver.c
+++ b/src/dummy_driver.c
@@ -391,7 +391,7 @@ DUMMYPreInit(ScrnInfoPtr pScrn, int flags)
     clockRanges->next = NULL;
     clockRanges->ClockMulFactor = 1;
     clockRanges->minClock = 11000;   /* guessed §§§ */
-    clockRanges->maxClock = 300000;
+    clockRanges->maxClock = maxClock;
     clockRanges->clockIndex = -1;              /* programmable */
     clockRanges->interlaceAllowed = TRUE;
     clockRanges->doubleScanAllowed = TRUE;

Note: this would change the default from 300000 to 230000 (different
values hardcoded in two places).
To keep the current default, just update the (currently unused) value
defined at the top of the function to read:
int maxClock = 300000;

Please apply. (I would rather avoid the git pull dance for this trivial fix)

Cheers
Antoine