drm/amdgpu: update documentation of parameter amdgpu_gtt_size

Fixes: f7ba887f ("drm/amdgpu: Adjust logic around GTT size (v3)")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: ZhenGuo Yin <zhenguo.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c

@@ -238,8 +238,7 @@ module_param_named(gartsize, amdgpu_gart_size, uint, 0600);
 
 /**
  * DOC: gttsize (int)
- * Restrict the size of GTT domain in MiB for testing. The default is -1 (It's VRAM size if 3GB < VRAM < 3/4 RAM,
- * otherwise 3/4 RAM size).
+ * Restrict the size of GTT domain in MiB for testing. The default is -1 (Use 1/2 RAM, minimum value is 3GB).
  */
 MODULE_PARM_DESC(gttsize, "Size of the GTT domain in megabytes (-1 = auto)");
 module_param_named(gttsize, amdgpu_gtt_size, int, 0600);