agp: add missing MODULE_DESCRIPTION() macros

make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/char/agp/amd64-agp.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/char/agp/intel-agp.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/char/agp/intel-gtt.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/char/agp/sis-agp.o
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/char/agp/via-agp.o

Add the missing invocations of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240603-md-agp-v1-1-9a1582114ced@quicinc.com