Fix return value check of drmIoctl()

Merged Niclas Zeising requested to merge zeising/xf86-video-amdgpu:zeising/drmioctl into master

When the drmModeSetCursor2() call was replaced with bare drmIoctl() call in b344e155, a bug was introduced. With the use of drmModeSetCursor2(), the return value from drmIoctl() (which calls ioctl()) were mangled, if they were negative, they were replaced by -errno by a wrapper function in xf86drMode.c in libdrm. After replacing drmModeSetCursor2() with the call to drmIoctl(), this mangling no longer happens, and we need to explicitly check if the call to drmIoctl() fails, which is indicated by returning -1, and then why it failed, by checking errno. If the error indicated by errno is EINVAL, then we can't use the DRM_IOCTL_MODE_CURSOR2 ioctl(), and need to fall back to the DRM_IOCTL_MODE_CURSOR ioctl().

This bug can manifest itself by an invisible hw cursor on systems where the DRM_IOCTL_MODE_CURSOR2 is not implemented by the graphics driver.

This fix is analogous to the following fix for xf86-video-ati: xf86-video-ati!4 (merged)

Signed-off-by: Niclas Zeising

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