drm/amdkfd: Remove DUMMY_VRAM_SIZE

Remove DUMMY_VRAM_SIZE as it is not needed and can result
in reporting incorrect memory size.

Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>