drm: add interface to get drm devices on the system v3

For mutiple GPU support, the devices on the system should be enumerated
to get necessary information about each device, and the drmGetDevices
interface is added for this. Currently only PCI devices are supported for
the enumeration.

Typical usage:
int count;
drmDevicePtr *foo;
count = drmGetDevices(NULL, 0);
foo = calloc(count, sizeof(drmDevicePtr));
count = drmGetDevices(foo, count);
/* find proper device, open correct device node, etc */
drmFreeDevices(foo, count);
free(foo);

v2: [Jammy Zhou]
 - return a list of devices, rather than nodes
v3: [Jammy Zhou]
 - fix the signed extension for PCI device info

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Jammy Zhou <Jammy.Zhou@amd.com>