Don't call xf86DisableRandR in ABI_VIDEODRV_VERSION 24 & later

The API was removed by Xserver commit dd00e5466a0e4ea313d1860824da4123692827ed in xorg-server-1.20.0 and later.

Found by gcc -Werror=implicit:

riva_driver.c: In function ‘RivaScreenInit’:
riva_driver.c:1213:12: error: implicit declaration of function ‘xf86DisableRandR’; did you mean ‘xf86DisableIO’? [-Werror=implicit-function-declaration]
 1213 |            xf86DisableRandR();
      |            ^~~~~~~~~~~~~~~~
      |            xf86DisableIO
nv_driver.c: In function ‘NVScreenInit’:
nv_driver.c:2635:16: error: implicit declaration of function ‘xf86DisableRandR’; did you mean ‘xf86DisableIO’? [-Werror=implicit-function-declaration]
 2635 |                xf86DisableRandR();
      |                ^~~~~~~~~~~~~~~~
      |                xf86DisableIO

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merged